Although almost everyone is well-served by the defaults, some uses of RCU
benefit from shorter grace periods, while others benefit more from the
greater efficiency provided by longer grace periods. Situations requiring
a large number of grace periods to elapse (and wireshark startup has
been called out as an example of this) are helped by lower-latency
grace periods. Furthermore, in some embedded applications, people are
willing to accept a small degradation in update efficiency (due to there
being more of the shorter grace-period operations) in order to gain the
lower latency.
In contrast, those few systems with thousands of CPUs need longer grace
periods because the CPU overhead of a grace period rises roughly
linearly with the number of CPUs. Such systems normally do not make
much use of facilities that require large numbers of grace periods to
elapse, so this is a good tradeoff.
Therefore, this commit allows the durations to be controlled from sysfs.
There are two sysfs parameters, one named "jiffies_till_first_fqs" that
specifies the delay in jiffies from the end of grace-period initialization
until the first attempt to force quiescent states, and the other named
"jiffies_till_next_fqs" that specifies the delay (again in jiffies)
between subsequent attempts to force quiescent states. They both default
to three jiffies, which is compatible with the old hard-coded behavior.
At some future time, it may be possible to automatically increase the
grace-period length with the number of CPUs, but we do not yet have
sufficient data to do a good job. Preliminary data indicates that we
should add an addiitonal jiffy to each of the delays for every 200 CPUs
in the system, but more experimentation is needed. For now, the number
of systems with more than 1,000 CPUs is small enough that this can be
relegated to boot-time hand tuning.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Large systems running RCU_FAST_NO_HZ kernels see extreme memory
contention on the rcu_state structure's ->fqslock field. This
can be avoided by disabling RCU_FAST_NO_HZ, either at compile time
or at boot time (via the nohz kernel boot parameter), but large
systems will no doubt become sensitive to energy consumption.
This commit therefore uses a combining-tree approach to spread the
memory contention across new cache lines in the leaf rcu_node structures.
This can be thought of as a tournament lock that has only a try-lock
acquisition primitive.
The effect on small systems is minimal, because such systems have
an rcu_node "tree" consisting of a single node. In addition, this
functionality is not used on fastpaths.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
RCU quiescent-state forcing is currently carried out without preemption
points, which can result in excessive latency spikes on large systems
(many hundreds or thousands of CPUs). This patch therefore inserts
a voluntary preemption point into force_qs_rnp(), which should greatly
reduce the magnitude of these spikes.
Reported-by: Mike Galbraith <mgalbraith@suse.de>
Reported-by: Dimitri Sivanich <sivanich@sgi.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
As the first step towards allowing quiescent-state forcing to be
preemptible, this commit moves RCU quiescent-state forcing into the
same kthread that is now used to initialize and clean up after grace
periods. This is yet another step towards keeping scheduling
latency down to a dull roar.
Updated to change from raw_spin_lock_irqsave() to raw_spin_lock_irq()
and to remove the now-unused rcu_state structure fields as suggested by
Peter Zijlstra.
Reported-by: Mike Galbraith <mgalbraith@suse.de>
Reported-by: Dimitri Sivanich <sivanich@sgi.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Earlier versions of RCU invoked the RCU core from the CPU_DYING notifier
in order to note a quiescent state for the outgoing CPU. Because the
CPU is marked "offline" during the execution of the CPU_DYING notifiers,
the RCU core had to tolerate being invoked from an offline CPU. However,
commit b1420f1c (Make rcu_barrier() less disruptive) left only tracing
code in the CPU_DYING notifier, so the RCU core need no longer execute
on offline CPUs. This commit therefore enforces this restriction.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Then rcu_gp_kthread() function is too large and furthermore needs to
have the force_quiescent_state() code pulled in. This commit therefore
breaks up rcu_gp_kthread() into rcu_gp_init() and rcu_gp_cleanup().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
RCU grace-period cleanup is currently carried out with interrupts
disabled, which can result in excessive latency spikes on large systems
(many hundreds or thousands of CPUs). This patch therefore makes the
RCU grace-period cleanup be preemptible, including voluntary preemption
points, which should eliminate those latency spikes. Similar spikes from
forcing of quiescent states will be dealt with similarly by later patches.
Updated to replace uses of spin_lock_irqsave() with spin_lock_irq(), as
suggested by Peter Zijlstra.
Reported-by: Mike Galbraith <mgalbraith@suse.de>
Reported-by: Dimitri Sivanich <sivanich@sgi.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
As a first step towards allowing grace-period cleanup to be preemptible,
this commit moves the RCU grace-period cleanup into the same kthread
that is now used to initialize grace periods. This is needed to keep
scheduling latency down to a dull roar.
[ paulmck: Get rid of stray spin_lock_irqsave() calls. ]
Reported-by: Mike Galbraith <mgalbraith@suse.de>
Reported-by: Dimitri Sivanich <sivanich@sgi.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
RCU grace-period initialization is currently carried out with interrupts
disabled, which can result in 200-microsecond latency spikes on systems
on which RCU has been configured for 4096 CPUs. This patch therefore
makes the RCU grace-period initialization be preemptible, which should
eliminate those latency spikes. Similar spikes from grace-period cleanup
and the forcing of quiescent states will be dealt with similarly by later
patches.
Reported-by: Mike Galbraith <mgalbraith@suse.de>
Reported-by: Dimitri Sivanich <sivanich@sgi.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
The next step in reducing RCU's grace-period initialization latency on
large systems will make this initialization preemptible. Unfortunately,
making the grace-period initialization subject to interrupts (let alone
preemption) exposes the following race on systems whose rcu_node tree
contains more than one node:
1. CPU 31 starts initializing the grace period, including the
first leaf rcu_node structures, and is then preempted.
2. CPU 0 refers to the first leaf rcu_node structure, and notes
that a new grace period has started. It passes through a
quiescent state shortly thereafter, and informs the RCU core
of this rite of passage.
3. CPU 0 enters an RCU read-side critical section, acquiring
a pointer to an RCU-protected data item.
4. CPU 31 takes an interrupt whose handler removes the data item
referenced by CPU 0 from the data structure, and registers an
RCU callback in order to free it.
5. CPU 31 resumes initializing the grace period, including its
own rcu_node structure. In invokes rcu_start_gp_per_cpu(),
which advances all callbacks, including the one registered
in #4 above, to be handled by the current grace period.
6. The remaining CPUs pass through quiescent states and inform
the RCU core, but CPU 0 remains in its RCU read-side critical
section, still referencing the now-removed data item.
7. The grace period completes and all the callbacks are invoked,
including the one that frees the data item that CPU 0 is still
referencing. Oops!!!
One way to avoid this race is to remove grace-period acceleration from
rcu_start_gp_per_cpu(). Now, the only reason for this acceleration was
to allow CPUs bringing RCU out of idle state to have their callbacks
invoked after only one grace period, rather than the two grace periods
that would otherwise be required. But this acceleration does not
work when RCU grace-period initialization is moved to a kthread because
the CPU posting the callback is no longer necessarily the CPU that is
initializing the resulting grace period.
This commit therefore removes this now-pointless (and soon to be dangerous)
grace-period acceleration, thus avoiding the above race.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
As the first step towards allowing grace-period initialization to be
preemptible, this commit moves the RCU grace-period initialization
into its own kthread. This is needed to keep large-system scheduling
latency at reasonable levels.
Also change raw_spin_lock_irqsave() to raw_spin_lock_irq() as suggested
by Peter Zijlstra in review comments.
Reported-by: Mike Galbraith <mgalbraith@suse.de>
Reported-by: Dimitri Sivanich <sivanich@sgi.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Each grace period is supposed to have at least one callback waiting
for that grace period to complete. However, if CONFIG_NO_HZ=n, an
extra callback-free grace period is no big problem -- it will chew up
a tiny bit of CPU time, but it will complete normally. In contrast,
CONFIG_NO_HZ=y kernels have the potential for all the CPUs to go to
sleep indefinitely, in turn indefinitely delaying completion of the
callback-free grace period. Given that nothing is waiting on this grace
period, this is also not a problem.
That is, unless RCU CPU stall warnings are also enabled, as they are
in recent kernels. In this case, if a CPU wakes up after at least one
minute of inactivity, an RCU CPU stall warning will result. The reason
that no one noticed until quite recently is that most systems have enough
OS noise that they will never remain absolutely idle for a full minute.
But there are some embedded systems with cut-down userspace configurations
that consistently get into this situation.
All this begs the question of exactly how a callback-free grace period
gets started in the first place. This can happen due to the fact that
CPUs do not necessarily agree on which grace period is in progress.
If a CPU still believes that the grace period that just completed is
still ongoing, it will believe that it has callbacks that need to wait for
another grace period, never mind the fact that the grace period that they
were waiting for just completed. This CPU can therefore erroneously
decide to start a new grace period. Note that this can happen in
TREE_RCU and TREE_PREEMPT_RCU even on a single-CPU system: Deadlock
considerations mean that the CPU that detected the end of the grace
period is not necessarily officially informed of this fact for some time.
Once this CPU notices that the earlier grace period completed, it will
invoke its callbacks. It then won't have any callbacks left. If no
other CPU has any callbacks, we now have a callback-free grace period.
This commit therefore makes CPUs check more carefully before starting a
new grace period. This new check relies on an array of tail pointers
into each CPU's list of callbacks. If the CPU is up to date on which
grace periods have completed, it checks to see if any callbacks follow
the RCU_DONE_TAIL segment, otherwise it checks to see if any callbacks
follow the RCU_WAIT_TAIL segment. The reason that this works is that
the RCU_WAIT_TAIL segment will be promoted to the RCU_DONE_TAIL segment
as soon as the CPU is officially notified that the old grace period
has ended.
This change is to cpu_needs_another_gp(), which is called in a number
of places. The only one that really matters is in rcu_start_gp(), where
the root rcu_node structure's ->lock is held, which prevents any
other CPU from starting or completing a grace period, so that the
comparison that determines whether the CPU is missing the completion
of a grace period is stable.
Reported-by: Becky Bruce <bgillbruce@gmail.com>
Reported-by: Subodh Nijsure <snijsure@grid-net.com>
Reported-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Paul Walmsley <paul@pwsan.com> # OMAP3730, OMAP4430
Cc: stable@vger.kernel.org
Bring RCU into the new-age CPU-hotplug fold by modifying RCU's per-CPU
kthread code to use the new smp_hotplug_thread facility.
[ tglx: Adapted it to use callbacks and to the simplified rcu yield ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20120716103948.673354828@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
The rcu_yield() code is amazing. It's there to avoid starvation of the
system when lots of (boosting) work is to be done.
Now looking at the code it's functionality is:
Make the thread SCHED_OTHER and very nice, i.e. get it out of the way
Arm a timer with 2 ticks
schedule()
Now if the system goes idle the rcu task returns, regains SCHED_FIFO
and plugs on. If the systems stays busy the timer fires and wakes a
per node kthread which in turn makes the per cpu thread SCHED_FIFO and
brings it back on the cpu. For the boosting thread the "make it FIFO"
bit is missing and it just runs some magic boost checks. Now this is a
lot of code with extra threads and complexity.
It's way simpler to let the tasks when they detect overload schedule
away for 2 ticks and defer the normal wakeup as long as they are in
yielded state and the cpu is not idle.
That solves the same problem and the only difference is that when the
cpu goes idle it's not guaranteed that the thread returns right away,
but it won't be longer out than two ticks, so no harm is done. If
that's an issue than it is way simpler just to wake the task from
idle as RCU has callbacks there anyway.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Namhyung Kim <namhyung@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20120716103948.131256723@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
The Linux kernel coding style says that single-statement blocks should
omit curly braces unless the other leg of the "if" statement has
multiple statements, in which case the curly braces should be included.
This commit fixes RCU's violations of this rule.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
bigrtm: First steps towards getting RCU out of the way of
tens-of-microseconds real-time response on systems compiled
with NR_CPUS=4096. Also cleanups for and increased concurrency
of rcu_barrier() family of primitives.
doctorture: rcutorture and documentation improvements.
fixes: Miscellaneous fixes.
fnh: RCU_FAST_NO_HZ fixes and improvements.
The recent bug that introduced the RCU callback list/count mismatch
showed the need for a diagnostic to check for this, which this commit
adds.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Problems in RCU idle entry and exit are almost always confined to the
offending CPU. This commit therefore switches ftrace_dump() from
DUMP_ALL to DUMP_ORIG.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Tested-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr>
If a CPU goes offline with callbacks queued, those callbacks might be
indefinitely postponed, which can result in a system hang. This commit
therefore inserts warnings for this condition.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
It is time to optimize CONFIG_TREE_PREEMPT_RCU's synchronize_rcu()
for uniprocessor optimization, which means that rcu_blocking_is_gp()
can no longer rely on RCU read-side critical sections having disabled
preemption. This commit therefore disables preemption across
rcu_blocking_is_gp()'s scan of the cpu_online_mask.
(Updated from previous version to fix embarrassing bug spotted by
Wu Fengguang.)
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The __call_rcu() function is a bit overweight, so this commit splits
it into actual enqueuing of and accounting for the callback (__call_rcu())
and associated RCU-core processing (__call_rcu_core()).
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
The __call_rcu() function will invoke the RCU core, for example, if
it detects that the current CPU has too many callbacks. However, this
can happen on an offline CPU that is on its way to the idle loop, in
which case it is an error to invoke the RCU core, and the excess callbacks
will be adopted in any case. This commit therefore adds checks to
__call_rcu() for running on an offline CPU, refraining from invoking
the RCU core in this case.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Although __call_rcu() is handled correctly when called from a momentary
non-idle period, if it is called on a CPU that RCU believes to be idle
on RCU_FAST_NO_HZ kernels, the callback might be indefinitely postponed.
This commit therefore ensures that RCU is aware of the new callback and
has a chance to force the CPU out of dyntick-idle mode when a new callback
is posted.
Reported-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
The _rcu_barrier() function accesses other CPUs' rcu_data structure's
->qlen field without benefit of locking. This commit therefore adds
the required ACCESS_ONCE() wrappers around accesses and updates that
need it.
ACCESS_ONCE() is not needed when a CPU accesses its own ->qlen, or
in code that cannot run while _rcu_barrier() is sampling ->qlen fields.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
There are a couple of open-coded initializations of the rcu_data
structure's RCU callback list. This commit therefore consolidates
them into a new init_callback_list() function.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
The code that attempts to identify stalls that end just as we detect
them is broken by both flavors of initialization failure. This commit
therefore properly initializes and computes the count of the number
of reasons why the RCU grace period is stalled.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Before RCU had unified idle, the RCU_SAVE_DYNTICK leg of the switch
statement in force_quiescent_state() was dead code for CONFIG_NO_HZ=n
kernel builds. With unified idle, the code is never dead. This commit
therefore removes the "if" statement designed to make gcc aware of when
the code was and was not dead.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
The arrival of TREE_PREEMPT_RCU some years back included some ugly
code involving either #ifdef or #ifdef'ed wrapper functions to iterate
over all non-SRCU flavors of RCU. This commit therefore introduces
a for_each_rcu_flavor() iterator over the rcu_state structures for each
flavor of RCU to clean up a bit of the ugliness.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
With the advent of __this_cpu_ptr(), it is no longer necessary to pass
both the rcu_state and rcu_data structures into __rcu_process_callbacks().
This commit therefore computes the rcu_data pointer from the rcu_state
pointer within __rcu_process_callbacks() so that callers can pass in
only the pointer to the rcu_state structure. This paves the way for
linking the rcu_state structures together and iterating over them.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
This commit adds event tracing for _rcu_barrier() execution. This
is defined only if RCU_TRACE=y.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
The traditional rcu_barrier() implementation has serialized all requests,
regardless of RCU flavor, and also does not coalesce concurrent requests.
In the past, this has been good and sufficient.
However, systems are getting larger and use of rcu_barrier() has been
increasing. This commit therefore introduces a counter-based scheme
that allows _rcu_barrier() calls for the same flavor of RCU to take
advantage of each others' work.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
For global variables, C defaults all fields to zero. The initialization
of the rcu_state structure's ->n_force_qs and ->n_force_qs_ngp fields
is therefore redundant, so this commit removes these initializations.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
In order to allow each RCU flavor to concurrently execute its
rcu_barrier() function, it is necessary to move the relevant
state to the rcu_state structure. This commit therefore moves the
rcu_barrier_mutex global variable to a new ->barrier_mutex field
in the rcu_state structure.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
In order to allow each RCU flavor to concurrently execute its
rcu_barrier() function, it is necessary to move the relevant
state to the rcu_state structure. This commit therefore moves the
rcu_barrier_completion global variable to a new ->barrier_completion
field in the rcu_state structure.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
In order to allow each RCU flavor to concurrently execute its rcu_barrier()
function, it is necessary to move the relevant state to the rcu_state
structure. This commit therefore moves the rcu_barrier_cpu_count global
variable to a new ->barrier_cpu_count field in the rcu_state structure.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
In order for multiple flavors of RCU to each concurrently run one
rcu_barrier(), each flavor needs its own per-CPU set of rcu_head
structures. This commit therefore moves _rcu_barrier()'s set of
per-CPU rcu_head structures from per-CPU variables to the existing
per-CPU and per-RCU-flavor rcu_data structures.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
This is a preparatory commit for increasing rcu_barrier()'s concurrency.
It adds a pointer in the rcu_data structure to the corresponding call_rcu()
function. This allows a pointer to the rcu_data structure to imply the
function pointer, which allows _rcu_barrier() state to be placed in the
rcu_state structure.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Upcoming rcu_barrier() concurrency commits will result in line lengths
greater than 80 characters in the RCU_STATE_INITIALIZER(), so this commit
shortens the name of the macro's argument to prevent this.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
The rcu_node tree array is sized based on compile-time constants,
including NR_CPUS. Although this approach has worked well in the past,
the recent trend by many distros to define NR_CPUS=4096 results in
excessive grace-period-initialization latencies.
This commit therefore substitutes the run-time computed nr_cpu_ids for
the compile-time NR_CPUS when building the tree. This can result in
much of the compile-time-allocated rcu_node array being unused. If
this is a major problem, you are in a specialized situation anyway,
so you can manually adjust the NR_CPUS, RCU_FANOUT, and RCU_FANOUT_LEAF
kernel config parameters.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Although making RCU_FANOUT_LEAF a kernel configuration parameter rather
than a fixed constant makes it easier for people to decrease cache-miss
overhead for large systems, it is of little help for people who must
run a single pre-built kernel binary.
This commit therefore allows the value of RCU_FANOUT_LEAF to be
increased (but not decreased!) via a boot-time parameter named
rcutree.rcu_fanout_leaf.
Reported-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This reverts commit 616c310e83.
(Move PREEMPT_RCU preemption to switch_to() invocation).
Testing by Sasha Levin <levinsasha928@gmail.com> showed that this
can result in deadlock due to invoking the scheduler when one of
the runqueue locks is held. Because this commit was simply a
performance optimization, revert it.
Reported-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Sasha Levin <levinsasha928@gmail.com>
Commit b1420f1c (Make rcu_barrier() less disruptive) rearranged the
code in rcu_do_batch(), moving the ->qlen manipulation to follow
the requeueing of the callbacks. Unfortunately, this rearrangement
clobbered the value of the "count" local variable before the value
of rdp->qlen was adjusted, resulting in the value of rdp->qlen being
inaccurate. This commit therefore introduces an index variable "i",
avoiding the inadvertent multiplexing.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
In the present implementations of CPU hotplug, the outgoing CPU is
guaranteed to run its stop-machine process on the way out, which
will guarantee that RCU_FAST_NO_HZ forces the CPU out of dyntick-idle
mode.
However, new versions of CPU hotplug might not work this way. This
commit therefore removes this design constraint by explicitly notifying
CPUs when they adopt non-lazy RCU callbacks.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Tested-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr>
Pull the v3.5 RCU tree from Paul E. McKenney:
1) A set of improvements and fixes to the RCU_FAST_NO_HZ feature
(with more on the way for 3.6). Posted to LKML:
https://lkml.org/lkml/2012/4/23/324 (commits 1-3 and 5),
https://lkml.org/lkml/2012/4/16/611 (commit 4),
https://lkml.org/lkml/2012/4/30/390 (commit 6), and
https://lkml.org/lkml/2012/5/4/410 (commit 7, combined with
the other commits for the convenience of the tester).
2) Changes to make rcu_barrier() avoid disrupting execution of CPUs
that have no RCU callbacks. Posted to LKML:
https://lkml.org/lkml/2012/4/23/322.
3) A couple of commits that improve the efficiency of the interaction
between preemptible RCU and the scheduler, these two being all
that survived an abortive attempt to allow preemptible RCU's
__rcu_read_lock() to be inlined. The full set was posted to
LKML at https://lkml.org/lkml/2012/4/14/143, and the first and
third patches of that set remain.
4) Lai Jiangshan's algorithmic implementation of SRCU, which includes
call_srcu() and srcu_barrier(). A major feature of this new
implementation is that synchronize_srcu() no longer disturbs
the execution of other CPUs. This work is based on earlier
implementations by Peter Zijlstra and Paul E. McKenney. Posted to
LKML: https://lkml.org/lkml/2012/2/22/82.
5) A number of miscellaneous bug fixes and improvements which were
posted to LKML at: https://lkml.org/lkml/2012/4/23/353 with
subsequent updates posted to LKML.
Signed-off-by: Ingo Molnar <mingo@kernel.org>
barrier: Reduce the amount of disturbance by rcu_barrier() to the rest of
the system. This branch also includes improvements to
RCU_FAST_NO_HZ, which are included here due to conflicts.
fixes: Miscellaneous fixes.
inline: Remaining changes from an abortive attempt to inline
preemptible RCU's __rcu_read_lock(). These are (1) making
exit_rcu() avoid unnecessary work and (2) avoiding having
preemptible RCU record a blocked thread when the scheduler
declines to do a context switch.
srcu: Lai Jiangshan's algorithmic implementation of SRCU, including
call_srcu().
The rcu_barrier() primitive interrupts each and every CPU, registering
a callback on every CPU. Once all of these callbacks have been invoked,
rcu_barrier() knows that every callback that was registered before
the call to rcu_barrier() has also been invoked.
However, there is no point in registering a callback on a CPU that
currently has no callbacks, most especially if that CPU is in a
deep idle state. This commit therefore makes rcu_barrier() avoid
interrupting CPUs that have no callbacks. Doing this requires reworking
the handling of orphaned callbacks, otherwise callbacks could slip through
rcu_barrier()'s net by being orphaned from a CPU that rcu_barrier() had
not yet interrupted to a CPU that rcu_barrier() had already interrupted.
This reworking was needed anyway to take a first step towards weaning
RCU from the CPU_DYING notifier's use of stop_cpu().
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Currently, PREEMPT_RCU readers are enqueued upon entry to the scheduler.
This is inefficient because enqueuing is required only if there is a
context switch, and entry to the scheduler does not guarantee a context
switch.
The commit therefore moves the enqueuing to immediately precede the
call to switch_to() from the scheduler.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Linus Torvalds <torvalds@linux-foundation.org>
Both Steven Rostedt's new idle-capable trace macros and the RCU_NONIDLE()
macro can cause RCU to momentarily pause out of idle without the rest
of the system being involved. This can cause rcu_prepare_for_idle()
to run through its state machine too quickly, which can in turn result
in needless scheduling-clock interrupts.
This commit therefore adds code to enable rcu_prepare_for_idle() to
distinguish between an initial entry to idle on the one hand (which needs
to advance the rcu_prepare_for_idle() state machine) and an idle reentry
due to idle-capable trace macros and RCU_NONIDLE() on the other hand
(which should avoid advancing the rcu_prepare_for_idle() state machine).
Additional state is maintained to allow the timer to be correctly reposted
when returning after a momentary pause out of idle, and even more state
is maintained to detect when new non-lazy callbacks have been enqueued
(which may require re-evaluation of the approach to idleness).
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The rcu_blocking_is_gp() function tests to see if there is only one
online CPU, and if so, synchronize_sched() and friends become no-ops.
However, for larger systems, num_online_cpus() scans a large vector,
and might be preempted while doing so. While preempted, any number
of CPUs might come online and go offline, potentially resulting in
num_online_cpus() returning 1 when there never had only been one
CPU online. This could result in a too-short RCU grace period, which
could in turn result in total failure, except that the only way that
the grace period is too short is if there is an RCU read-side critical
section spanning it. For RCU-sched and RCU-bh (which are the only
cases using rcu_blocking_is_gp()), RCU read-side critical sections
have either preemption or bh disabled, which prevents CPUs from going
offline. This in turn prevents actual failures from occurring.
This commit therefore adds a large block comment to rcu_blocking_is_gp()
documenting why it is safe. This commit also moves rcu_blocking_is_gp()
into kernel/rcutree.c, which should help prevent unwary developers from
mistaking it for a generally useful function.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Commit #0209f649 (rcu: limit rcu_node leaf-level fanout) set an upper
limit of 16 on the leaf-level fanout for the rcu_node tree. This was
needed to reduce lock contention that was induced by the synchronization
of scheduling-clock interrupts, which was in turn needed to improve
energy efficiency for moderate-sized lightly loaded servers.
However, reducing the leaf-level fanout means that there are more
leaf-level rcu_node structures in the tree, which in turn means that
RCU's grace-period initialization incurs more cache misses. This is
not a problem on moderate-sized servers with only a few tens of CPUs,
but becomes a major source of real-time latency spikes on systems with
many hundreds of CPUs. In addition, the workloads running on these large
systems tend to be CPU-bound, which eliminates the energy-efficiency
advantages of synchronizing scheduling-clock interrupts. Therefore,
these systems need maximal values for the rcu_node leaf-level fanout.
This commit addresses this problem by introducing a new kernel parameter
named RCU_FANOUT_LEAF that directly controls the leaf-level fanout.
This parameter defaults to 16 to handle the common case of a moderate
sized lightly loaded servers, but may be set higher on larger systems.
Reported-by: Mike Galbraith <efault@gmx.de>
Reported-by: Dimitri Sivanich <sivanich@sgi.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
As of:
29494be71a ("rcu,cleanup: simplify the code when cpu is dying")
RCU adopts callbacks from the dying CPU in its CPU_DYING notifier,
which means that any callbacks posted by later CPU_DYING notifiers
are ignored until the CPU comes back online.
A WARN_ON_ONCE() was added to __call_rcu() by:
e560140008 ("rcu: Simplify offline processing")
to check for this condition. Although this condition did not trigger
(at least as far as I know) during -next testing, it did recently
trigger in mainline:
https://lkml.org/lkml/2012/4/2/34
What is needed longer term is for RCU's CPU_DEAD notifier to adopt any
callbacks that were posted by CPU_DYING notifiers, however, the Linux
kernel has been running with this sort of thing happening for quite
some time. So the only thing that qualifies as a regression is the
WARN_ON_ONCE(), which this commit removes.
Making RCU's CPU_DEAD notifier adopt callbacks posted by CPU_DYING
notifiers is a topic for the 3.5 release of the Linux kernel.
Reported-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
synchronize_sched_expedited() is spamming CONFIG_DEBUG_PREEMPT=y
users with an unintended warning from the cpu_is_offline() check: use
raw_smp_processor_id() instead of smp_processor_id() there.
Because the warning is under a get_online_cpus(), it is not possible
for any CPUs to go offline, though it is quite possible that the
task might migrate between the raw_smp_processor_id() and the check
of cpu_is_offline(). This is not a problem because the task cannot
migrate from an offline CPU to an online one or vice versa. The point
of the check is to verify that synchronize_sched_expedited() is not
called from an offline CPU, for example, from a CPU_DYING notifier, or,
more important, from an outgoing CPU making its way from its CPU_DYING
notifiers to the idle loop.
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
RCU, RCU-bh, and RCU-sched read-side critical sections are forbidden
in the inner idle loop, that is, between the rcu_idle_enter() and the
rcu_idle_exit() -- RCU will happily ignore any such read-side critical
sections. However, things like powertop need tracepoints in the inner
idle loop.
This commit therefore provides an RCU_NONIDLE() macro that can be used to
wrap code in the idle loop that requires RCU read-side critical sections.
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Deepthi Dharwar <deepthi@linux.vnet.ibm.com>
Use of RCU in the idle loop is incorrect, quite a few instances of
just that have made their way into mainline, primarily event tracing.
The problem with RCU read-side critical sections on CPUs that RCU believes
to be idle is that RCU is completely ignoring the CPU, along with any
attempts and RCU read-side critical sections.
The approaches of eliminating the offending uses and of pushing the
definition of idle down beyond the offending uses have both proved
impractical. The new approach is to encapsulate offending uses of RCU
with rcu_idle_exit() and rcu_idle_enter(), but this requires nesting
for code that is invoked both during idle and and during normal execution.
Therefore, this commit modifies rcu_idle_enter() and rcu_idle_exit() to
permit nesting.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Deepthi Dharwar <deepthi@linux.vnet.ibm.com>
The expedited RCU primitives can be quite useful, but they have some
high costs as well. This commit updates and creates docbook comments
calling out the costs, and updates the RCU documentation as well.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Because newly offlined CPUs continue executing after completing the
CPU_DYING notifiers, they legitimately enter the scheduler and use
RCU while appearing to be offline. This calls for a more sophisticated
approach as follows:
1. RCU marks the CPU online during the CPU_UP_PREPARE phase.
2. RCU marks the CPU offline during the CPU_DEAD phase.
3. Diagnostics regarding use of read-side RCU by offline CPUs use
RCU's accounting rather than the cpu_online_map. (Note that
__call_rcu() still uses cpu_online_map to detect illegal
invocations within CPU_DYING notifiers.)
4. Offline CPUs are prevented from hanging the system by
force_quiescent_state(), which pays attention to cpu_online_map.
Some additional work (in a later commit) will be needed to
guarantee that force_quiescent_state() waits a full jiffy before
assuming that a CPU is offline, for example, when called from
idle entry. (This commit also makes the one-jiffy wait
explicit, since the old-style implicit wait can now be defeated
by RCU_FAST_NO_HZ and by rcutorture.)
This approach avoids the false positives encountered when attempting to
use more exact classification of CPU online/offline state.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Now that TREE_RCU and TREE_PREEMPT_RCU no longer do anything different
for the single-CPU case, there is no need for multiple definitions of
synchronize_sched_expedited(). It is no longer in any sense a plug-in,
so move it from kernel/rcutree_plugin.h to kernel/rcutree.c.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Although it is legal to use RCU during early boot, it is anything
but legal to use RCU at runtime from an offlined CPU. After all, RCU
explicitly ignores offlined CPUs. This commit therefore adds checks
for runtime use of RCU from offlined CPUs.
These checks are not perfect, in particular, they can be subverted
through use of things like rcu_dereference_raw(). Note that it is not
possible to put checks in rcu_read_lock() and friends due to the fact
that these primitives are used in code that might be used under either
RCU or lock-based protection, which means that checking rcu_read_lock()
gets you fat piles of false positives.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
There have been situations where RCU CPU stall warnings were caused by
issues in scheduling-clock timer initialization. To make it easier to
track these down, this commit causes the RCU CPU stall-warning messages
to print out the number of scheduling-clock interrupts taken in the
current grace period for each stalled CPU.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The default CONFIG_RCU_CPU_STALL_TIMEOUT value of 60 seconds has served
Linux users well for production use for quite some time. However, for
debugging, there will be more than three minutes between subsequent
stall-warning messages. This can be an annoyingly long wait if you
are trying to work out where the offending infinite loop is hiding.
Therefore, this commit provides a rcu_cpu_stall_timeout sysfs
parameter that may be adjusted at boot time and at runtime to speed
up debugging.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Now that both TINY_RCU and TINY_PREEMPT_RCU have been in place for awhile,
it is time to remove UP support from TREE_RCU, which is what this commit
does.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The inner idle loop is an extended quiescent state for all flavors
of RCU, but there have been recent bug involving use of RCU read-side
primitives from within the idle loop. Therefore, this commit enlists
lockdep-RCU to detect attempts to enter the inner idle loop while in
an RCU read-side critical section, emitting a lockdep-RCU splat if so.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The recent updates to RCU_CPU_FAST_NO_HZ have an rcu_needs_cpu() that
does more than just check for callbacks, so get the name for
rcu_preempt_needs_cpu() consistent with that change, now calling it
rcu_preempt_cpu_has_callbacks().
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The grace-period initialization sequence in rcu_start_gp() has a special
case for systems where the rcu_node tree is a single rcu_node structure.
This made sense some years ago when systems were smaller and up to 64
CPUs could share a single rcu_node structure, but now that large systems
are common and a given leaf rcu_node structure can support only 16 CPUs
(due to lock contention on the rcu_node's ->lock field), this optimization
is almost never taken. And even the small mobile platforms that might
make use of it might rather have the kernel text reduction.
Therefore, this commit removes the check for single-rcu_node trees.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
RCU's current CPU-offline code path dumps all of the outgoing CPU's
callbacks onto the RCU_NEXT_TAIL portion of the surviving CPU's
callback list. This means that all the ready-to-invoke callbacks from
the outgoing CPU must wait for another full RCU grace period. This was
just fine when CPU-hotplug events were rare, but there is increasing
evidence that users are planning to make increasing use of CPU hotplug.
Therefore, this commit changes the callback-dumping procedure so that
callbacks that are ready to invoke are moved to the RCU_DONE_TAIL
portion of the surviving CPU's callback list. This avoids running
these callbacks through a second unnecessary grace period.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Because quiescent states are now reported from offline CPUs in
CPU_DYING state, there is some possibility that such a CPU might
note the end of a grace period and attempt to start invoking
callbacks. This would be a very bad thing, and is supposed to
be prevented by the fact that the CPU_DYING CPU gets rid of all
its callbacks before reporting the quiescent state. However,
there is other CPU-offline code in the kernel, and it is quite
possible that someone will invoke RCU core processing from that
code. Therefore, this commit adds a warning for this case.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Move ->qsmaskinit and blkd_tasks[] manipulation to the CPU_DYING
notifier. This simplifies the code by eliminating a potential
deadlock and by reducing the responsibilities of force_quiescent_state().
Also rename functions to make their connection to the CPU-hotplug
stages explicit.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
When CONFIG_RCU_FAST_NO_HZ is enabled, RCU will allow a given CPU to
enter dyntick-idle mode even if it still has RCU callbacks queued.
RCU avoids system hangs in this case by scheduling a timer for several
jiffies in the future. However, if all of the callbacks on that CPU
are from kfree_rcu(), there is no reason to wake the CPU up, as it is
not a problem to defer freeing of memory.
This commit therefore tracks the number of callbacks on a given CPU
that are from kfree_rcu(), and avoids scheduling the timer if all of
a given CPU's callbacks are from kfree_rcu().
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The push for energy efficiency will require that RCU tag rcu_head
structures to indicate whether or not their invocation is time critical.
This tagging is best carried out in the bottom bits of the ->next
pointers in the rcu_head structures. This tagging requires that the
rcu_head structures be properly aligned, so this commit adds the required
diagnostics.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
It is illegal to have a grace period within a same-flavor RCU read-side
critical section, so this commit adds lockdep-RCU checks to splat when
such abuse is encountered. This commit does not detect more elaborate
RCU deadlock situations. These situations might be a job for lockdep
enhancements.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The current rcu_batch_end event trace records only the name of the RCU
flavor and the total number of callbacks that remain queued on the
current CPU. This is insufficient for testing and tuning the new
dyntick-idle RCU_FAST_NO_HZ code, so this commit adds idle state along
with whether or not any of the callbacks that were ready to invoke
at the beginning of rcu_do_batch() are still queued.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
No point in having two identical rcu_cpu_stall_suppress declarations,
so remove the more obscure of the two.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The rcu_do_batch() function that invokes callbacks for TREE_RCU and
TREE_PREEMPT_RCU normally throttles callback invocation to avoid degrading
scheduling latency. However, as long as the CPU would otherwise be idle,
there is no downside to continuing to invoke any callbacks that have passed
through their grace periods. In fact, processing such callbacks in a
timely manner has the benefit of increasing the probability that the
CPU can enter the power-saving dyntick-idle mode.
Therefore, this commit allows callback invocation to continue beyond the
preset limit as long as the scheduler does not have some other task to
run and as long as context is that of the idle task or the relevant
RCU kthread.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Because tasks don't nest, the ->dyntick_nesting must always be zero upon
entry to rcu_idle_enter_common(). Therefore, pass "0" rather than the
counter itself.
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Because tasks do not nest, rcu_idle_enter() and rcu_idle_exit() do
not need to check for nesting. This commit therefore moves nesting
checks from rcu_idle_enter_common() to rcu_irq_exit() and from
rcu_idle_exit_common() to rcu_irq_enter().
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The current implementation of RCU_FAST_NO_HZ prevents CPUs from entering
dyntick-idle state if they have RCU callbacks pending. Unfortunately,
this has the side-effect of often preventing them from entering this
state, especially if at least one other CPU is not in dyntick-idle state.
However, the resulting per-tick wakeup is wasteful in many cases: if the
CPU has already fully responded to the current RCU grace period, there
will be nothing for it to do until this grace period ends, which will
frequently take several jiffies.
This commit therefore permits a CPU that has done everything that the
current grace period has asked of it (rcu_pending() == 0) even if it
still as RCU callbacks pending. However, such a CPU posts a timer to
wake it up several jiffies later (6 jiffies, based on experience with
grace-period lengths). This wakeup is required to handle situations
that can result in all CPUs being in dyntick-idle mode, thus failing
to ever complete the current grace period. If a CPU wakes up before
the timer goes off, then it cancels that timer, thus avoiding spurious
wakeups.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Fixes and workarounds for a number of issues (for example, that in
df4012edc) make it safe to once again detect dyntick-idle CPUs on the
first pass of force_quiescent_state(), so this commit makes that change.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Assertions in rcu_init_percpu_data() unknowingly relied on outgoing
CPUs being turned off before reaching the idle loop. Unfortunately,
when running under kvm/qemu on x86, CPUs really can get to idle before
begin shut off. These CPUs are then born in dyntick-idle mode from an
RCU perspective, which results in splats in rcu_init_percpu_data() and
in RCU wrongly ignoring those CPUs despite them being active. This in
turn can cause RCU to end grace periods prematurely, potentially freeing
up memory that the newly onlined CPUs were still using. This is most
decidedly not what we need to see in an RCU implementation.
This commit therefore replaces the assertions in rcu_init_percpu_data()
with code that forces RCU's dyntick-idle view of newly onlined CPUs to
match reality.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
With the new implementation of RCU_FAST_NO_HZ, it was possible to hang
RCU grace periods as follows:
o CPU 0 attempts to go idle, cycles several times through the
rcu_prepare_for_idle() loop, then goes dyntick-idle when
RCU needs nothing more from it, while still having at least
on RCU callback pending.
o CPU 1 goes idle with no callbacks.
Both CPUs can then stay in dyntick-idle mode indefinitely, preventing
the RCU grace period from ever completing, possibly hanging the system.
This commit therefore prevents CPUs that have RCU callbacks from entering
dyntick-idle mode. This approach also eliminates the need for the
end-of-grace-period IPIs used previously.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Change from direct comparison of ->pid with zero to is_idle_task().
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
RCU has traditionally relied on idle_cpu() to determine whether a given
CPU is running in the context of an idle task, but commit 908a3283
(Fix idle_cpu()) has invalidated this approach. After commit 908a3283,
idle_cpu() will return true if the current CPU is currently running the
idle task, and will be doing so for the foreseeable future. RCU instead
needs to know whether or not the current CPU is currently running the
idle task, regardless of what the near future might bring.
This commit therefore switches from idle_cpu() to "current->pid != 0".
Reported-by: Wu Fengguang <fengguang.wu@intel.com>
Suggested-by: Carsten Emde <C.Emde@osadl.org>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Tested-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Currently, RCU does not permit a CPU to enter dyntick-idle mode if that
CPU has any RCU callbacks queued. This means that workloads for which
each CPU wakes up and does some RCU updates every few ticks will never
enter dyntick-idle mode. This can result in significant unnecessary power
consumption, so this patch permits a given to enter dyntick-idle mode if
it has callbacks, but only if that same CPU has completed all current
work for the RCU core. We determine use rcu_pending() to determine
whether a given CPU has completed all current work for the RCU core.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The current code just complains if the current task is not the idle task.
This commit therefore adds printing of the identity of the idle task.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
The trace_rcu_dyntick() trace event did not print both the old and
the new value of the nesting level, and furthermore printed only
the low-order 32 bits of it. This could result in some confusion
when interpreting trace-event dumps, so this commit prints both
the old and the new value, prints the full 64 bits, and also selects
the process-entry/exit increment to print nicely in hexadecimal.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Report that none of the rcu read lock maps are held while in an RCU
extended quiescent state (the section between rcu_idle_enter()
and rcu_idle_exit()). This helps detect any use of rcu_dereference()
and friends from within the section in idle where RCU is not allowed.
This way we can guarantee an extended quiescent window where the CPU
can be put in dyntick idle mode or can simply aoid to be part of any
global grace period completion while in the idle loop.
Uses of RCU from such mode are totally ignored by RCU, hence the
importance of these checks.
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
When setting up an expedited grace period, if there were no readers, the
task will awaken itself. This commit removes this useless self-awakening.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Because rcu_is_cpu_idle() is to be used to check for extended quiescent
states in RCU-preempt read-side critical sections, it cannot assume that
preemption is disabled. And preemption must be disabled when accessing
the dyntick-idle state, because otherwise the following sequence of events
could occur:
1. Task A on CPU 1 enters rcu_is_cpu_idle() and picks up the pointer
to CPU 1's per-CPU variables.
2. Task B preempts Task A and starts running on CPU 1.
3. Task A migrates to CPU 2.
4. Task B blocks, leaving CPU 1 idle.
5. Task A continues execution on CPU 2, accessing CPU 1's dyntick-idle
information using the pointer fetched in step 1 above, and finds
that CPU 1 is idle.
6. Task A therefore incorrectly concludes that it is executing in
an extended quiescent state, possibly issuing a spurious splat.
Therefore, this commit disables preemption within the rcu_is_cpu_idle()
function.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Earlier versions of RCU used the scheduling-clock tick to detect idleness
by checking for the idle task, but handled idleness differently for
CONFIG_NO_HZ=y. But there are now a number of uses of RCU read-side
critical sections in the idle task, for example, for tracing. A more
fine-grained detection of idleness is therefore required.
This commit presses the old dyntick-idle code into full-time service,
so that rcu_idle_enter(), previously known as rcu_enter_nohz(), is
always invoked at the beginning of an idle loop iteration. Similarly,
rcu_idle_exit(), previously known as rcu_exit_nohz(), is always invoked
at the end of an idle-loop iteration. This allows the idle task to
use RCU everywhere except between consecutive rcu_idle_enter() and
rcu_idle_exit() calls, in turn allowing architecture maintainers to
specify exactly where in the idle loop that RCU may be used.
Because some of the userspace upcall uses can result in what looks
to RCU like half of an interrupt, it is not possible to expect that
the irq_enter() and irq_exit() hooks will give exact counts. This
patch therefore expands the ->dynticks_nesting counter to 64 bits
and uses two separate bitfields to count process/idle transitions
and interrupt entry/exit transitions. It is presumed that userspace
upcalls do not happen in the idle loop or from usermode execution
(though usermode might do a system call that results in an upcall).
The counter is hard-reset on each process/idle transition, which
avoids the interrupt entry/exit error from accumulating. Overflow
is avoided by the 64-bitness of the ->dyntick_nesting counter.
This commit also adds warnings if a non-idle task asks RCU to enter
idle state (and these checks will need some adjustment before applying
Frederic's OS-jitter patches (http://lkml.org/lkml/2011/10/7/246).
In addition, validation of ->dynticks and ->dynticks_nesting is added.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
The ->signaled field was named before complications in the form of
dyntick-idle mode and offlined CPUs. These complications have required
that force_quiescent_state() be implemented as a state machine, instead
of simply unconditionally sending reschedule IPIs. Therefore, this
commit renames ->signaled to ->fqs_state to catch up with the new
force_quiescent_state() reality.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
The changed files were only including linux/module.h for the
EXPORT_SYMBOL infrastructure, and nothing else. Revector them
onto the isolated export header for faster compile times.
Nothing to see here but a whole lot of instances of:
-#include <linux/module.h>
+#include <linux/export.h>
This commit is only changing the kernel dir; next targets
will probably be mm, fs, the arch dirs, etc.
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
It is possible for the CPU that noted the end of the prior grace period
to not need a new one, and therefore to decide to propagate ->completed
throughout the rcu_node tree without starting another grace period.
However, in so doing, it releases the root rcu_node structure's lock,
which can allow some other CPU to start another grace period. The first
CPU will be propagating ->completed in parallel with the second CPU
initializing the rcu_node tree for the new grace period. In theory
this is harmless, but in practice we need to keep things simple.
This commit therefore moves the propagation of ->completed to
rcu_report_qs_rsp(), and refrains from marking the old grace period
as having been completed until it has finished doing this. This
prevents anyone from starting a new grace period concurrently with
marking the old grace period as having been completed.
Of course, the optimization where a CPU needing a new grace period
doesn't bother marking the old one completed is still in effect:
In that case, the marking happens implicitly as part of initializing
the new grace period.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The purpose of rcu_needs_cpu_flush() was to iterate on pushing the
current grace period in order to help the current CPU enter dyntick-idle
mode. However, this can result in failures if the CPU starts entering
dyntick-idle mode, but then backs out. In this case, the call to
rcu_pending() from rcu_needs_cpu_flush() might end up announcing a
non-existing quiescent state.
This commit therefore removes rcu_needs_cpu_flush() in favor of letting
the dyntick-idle machinery at the end of the softirq handler push the
loop along via its call to rcu_pending().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
RCU boost threads start life at RCU_BOOST_PRIO, while others remain
at RCU_KTHREAD_PRIO. While here, change thread names to match other
kthreads, and adjust rcu_yield() to not override the priority set by
the user. This last change sets the stage for runtime changes to
priority in the -rt tree.
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CPUs set rdp->qs_pending when coming online to resolve races with
grace-period start. However, this means that if RCU is idle, the
just-onlined CPU might needlessly send itself resched IPIs. Adjust
the online-CPU initialization to avoid this, and also to correctly
cause the CPU to respond to the current grace period if needed.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Josh Boyer <jwboyer@redhat.com>
Tested-by: Christian Hoffmann <email@christianhoffmann.info>
It is possible for an RCU CPU stall to end just as it is detected, in
which case the current code will uselessly dump all CPU's stacks.
This commit therefore checks for this condition and refrains from
sending needless NMIs.
And yes, the stall might also end just after we checked all CPUs and
tasks, but in that case we would at least have given some clue as
to which CPU/task was at fault.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Greater use of RCU during early boot (before the scheduler is operating)
is causing RCU to attempt to start grace periods during that time, which
in turn is resulting in both RCU and the callback functions attempting
to use the scheduler before it is ready.
This commit prevents these problems by prohibiting RCU grace periods
until after the scheduler has spawned the first non-idle task.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
There isn't a whole lot of point in poking the scheduler before there
are other tasks to switch to. This commit therefore adds a check
for rcu_scheduler_fully_active in __rcu_pending() to suppress any
pre-scheduler calls to set_need_resched(). The downside of this approach
is additional runtime overhead in a reasonably hot code path.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The trigger_all_cpu_backtrace() function is a no-op in architectures that
do not define arch_trigger_all_cpu_backtrace. On such architectures, RCU
CPU stall warning messages contain no stack trace information, which makes
debugging quite difficult. This commit therefore substitutes dump_stack()
for architectures that do not define arch_trigger_all_cpu_backtrace,
so that at least the local CPU's stack is dumped as part of the RCU CPU
stall warning message.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
When the ->dynticks field in the rcu_dynticks structure changed to an
atomic_t, its size on 64-bit systems changed from 64 bits to 32 bits.
The local variables in rcu_implicit_dynticks_qs() need to change as
well, hence this commit.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The in_irq() check in rcu_enter_nohz() is redundant because if we really
are in an interrupt, the attempt to re-enter dyntick-idle mode will invoke
rcu_needs_cpu() in any case, which will force the check for RCU callbacks.
So this commit removes the check along with the set_need_resched().
Suggested-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
