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rcu: Make rcu_barrier() less disruptive

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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>
This commit is contained in:
Paul E. McKenney 2012-03-01 13:18:08 -08:00 committed by Paul E. McKenney
parent 98248a0e24
commit b1420f1c8b
3 changed files with 222 additions and 88 deletions
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289
kernel/rcutree.c
View file

@ -75,6 +75,8 @@ static struct lock_class_key rcu_node_class[NUM_RCU_LVLS];
.gpnum = -300, \
.completed = -300, \
.onofflock = __RAW_SPIN_LOCK_UNLOCKED(&structname##_state.onofflock), \
.orphan_nxttail = &structname##_state.orphan_nxtlist, \
.orphan_donetail = &structname##_state.orphan_donelist, \
.fqslock = __RAW_SPIN_LOCK_UNLOCKED(&structname##_state.fqslock), \
.n_force_qs = 0, \
.n_force_qs_ngp = 0, \
@ -145,6 +147,13 @@ static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data *rdp);
unsigned long rcutorture_testseq;
unsigned long rcutorture_vernum;
/* State information for rcu_barrier() and friends. */
static DEFINE_PER_CPU(struct rcu_head, rcu_barrier_head) = {NULL};
static atomic_t rcu_barrier_cpu_count;
static DEFINE_MUTEX(rcu_barrier_mutex);
static struct completion rcu_barrier_completion;
/*
* Return true if an RCU grace period is in progress. The ACCESS_ONCE()s
* permit this function to be invoked without holding the root rcu_node
@ -1311,95 +1320,133 @@ rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp)
#ifdef CONFIG_HOTPLUG_CPU
/*
* Move a dying CPU's RCU callbacks to online CPU's callback list.
* Also record a quiescent state for this CPU for the current grace period.
* Synchronization and interrupt disabling are not required because
* this function executes in stop_machine() context. Therefore, cleanup
* operations that might block must be done later from the CPU_DEAD
* notifier.
*
* Note that the outgoing CPU's bit has already been cleared in the
* cpu_online_mask. This allows us to randomly pick a callback
* destination from the bits set in that mask.
* Send the specified CPU's RCU callbacks to the orphanage. The
* specified CPU must be offline, and the caller must hold the
* ->onofflock.
*/
static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
static void
rcu_send_cbs_to_orphanage(int cpu, struct rcu_state *rsp,
struct rcu_node *rnp, struct rcu_data *rdp)
{
int i;
unsigned long mask;
int receive_cpu = cpumask_any(cpu_online_mask);
struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
struct rcu_data *receive_rdp = per_cpu_ptr(rsp->rda, receive_cpu);
RCU_TRACE(struct rcu_node *rnp = rdp->mynode); /* For dying CPU. */
/* First, adjust the counts. */
/*
* Orphan the callbacks. First adjust the counts. This is safe
* because ->onofflock excludes _rcu_barrier()'s adoption of
* the callbacks, thus no memory barrier is required.
*/
if (rdp->nxtlist != NULL) {
receive_rdp->qlen_lazy += rdp->qlen_lazy;
receive_rdp->qlen += rdp->qlen;
rsp->qlen_lazy += rdp->qlen_lazy;
rsp->qlen += rdp->qlen;
rdp->n_cbs_orphaned += rdp->qlen;
rdp->qlen_lazy = 0;
rdp->qlen = 0;
}
/*
* Next, move ready-to-invoke callbacks to be invoked on some
* other CPU. These will not be required to pass through another
* grace period: They are done, regardless of CPU.
* Next, move those callbacks still needing a grace period to
* the orphanage, where some other CPU will pick them up.
* Some of the callbacks might have gone partway through a grace
* period, but that is too bad. They get to start over because we
* cannot assume that grace periods are synchronized across CPUs.
* We don't bother updating the ->nxttail[] array yet, instead
* we just reset the whole thing later on.
*/
if (rdp->nxtlist != NULL &&
rdp->nxttail[RCU_DONE_TAIL] != &rdp->nxtlist) {
struct rcu_head *oldhead;
struct rcu_head **oldtail;
struct rcu_head **newtail;
oldhead = rdp->nxtlist;
oldtail = receive_rdp->nxttail[RCU_DONE_TAIL];
rdp->nxtlist = *rdp->nxttail[RCU_DONE_TAIL];
*rdp->nxttail[RCU_DONE_TAIL] = *oldtail;
*receive_rdp->nxttail[RCU_DONE_TAIL] = oldhead;
newtail = rdp->nxttail[RCU_DONE_TAIL];
for (i = RCU_DONE_TAIL; i < RCU_NEXT_SIZE; i++) {
if (receive_rdp->nxttail[i] == oldtail)
receive_rdp->nxttail[i] = newtail;
if (rdp->nxttail[i] == newtail)
rdp->nxttail[i] = &rdp->nxtlist;
}
if (*rdp->nxttail[RCU_DONE_TAIL] != NULL) {
*rsp->orphan_nxttail = *rdp->nxttail[RCU_DONE_TAIL];
rsp->orphan_nxttail = rdp->nxttail[RCU_NEXT_TAIL];
*rdp->nxttail[RCU_DONE_TAIL] = NULL;
}
/*
* Finally, put the rest of the callbacks at the end of the list.
* The ones that made it partway through get to start over: We
* cannot assume that grace periods are synchronized across CPUs.
* (We could splice RCU_WAIT_TAIL into RCU_NEXT_READY_TAIL, but
* this does not seem compelling. Not yet, anyway.)
* Then move the ready-to-invoke callbacks to the orphanage,
* where some other CPU will pick them up. These will not be
* required to pass though another grace period: They are done.
*/
if (rdp->nxtlist != NULL) {
*receive_rdp->nxttail[RCU_NEXT_TAIL] = rdp->nxtlist;
receive_rdp->nxttail[RCU_NEXT_TAIL] =
rdp->nxttail[RCU_NEXT_TAIL];
receive_rdp->n_cbs_adopted += rdp->qlen;
rdp->n_cbs_orphaned += rdp->qlen;
*rsp->orphan_donetail = rdp->nxtlist;
rsp->orphan_donetail = rdp->nxttail[RCU_DONE_TAIL];
}
/* Finally, initialize the rcu_data structure's list to empty. */
rdp->nxtlist = NULL;
for (i = 0; i < RCU_NEXT_SIZE; i++)
rdp->nxttail[i] = &rdp->nxtlist;
}
}
/*
* Adopt the RCU callbacks from the specified rcu_state structure's
* orphanage. The caller must hold the ->onofflock.
*/
static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
{
int i;
struct rcu_data *rdp = __this_cpu_ptr(rsp->rda);
/*
* Record a quiescent state for the dying CPU. This is safe
* only because we have already cleared out the callbacks.
* (Otherwise, the RCU core might try to schedule the invocation
* of callbacks on this now-offline CPU, which would be bad.)
* If there is an rcu_barrier() operation in progress, then
* only the task doing that operation is permitted to adopt
* callbacks. To do otherwise breaks rcu_barrier() and friends
* by causing them to fail to wait for the callbacks in the
* orphanage.
*/
mask = rdp->grpmask; /* rnp->grplo is constant. */
if (rsp->rcu_barrier_in_progress &&
rsp->rcu_barrier_in_progress != current)
return;
/* Do the accounting first. */
rdp->qlen_lazy += rsp->qlen_lazy;
rdp->qlen += rsp->qlen;
rdp->n_cbs_adopted += rsp->qlen;
rsp->qlen_lazy = 0;
rsp->qlen = 0;
/*
* We do not need a memory barrier here because the only way we
* can get here if there is an rcu_barrier() in flight is if
* we are the task doing the rcu_barrier().
*/
/* First adopt the ready-to-invoke callbacks. */
if (rsp->orphan_donelist != NULL) {
*rsp->orphan_donetail = *rdp->nxttail[RCU_DONE_TAIL];
*rdp->nxttail[RCU_DONE_TAIL] = rsp->orphan_donelist;
for (i = RCU_NEXT_SIZE - 1; i >= RCU_DONE_TAIL; i--)
if (rdp->nxttail[i] == rdp->nxttail[RCU_DONE_TAIL])
rdp->nxttail[i] = rsp->orphan_donetail;
rsp->orphan_donelist = NULL;
rsp->orphan_donetail = &rsp->orphan_donelist;
}
/* And then adopt the callbacks that still need a grace period. */
if (rsp->orphan_nxtlist != NULL) {
*rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_nxtlist;
rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_nxttail;
rsp->orphan_nxtlist = NULL;
rsp->orphan_nxttail = &rsp->orphan_nxtlist;
}
}
/*
* Trace the fact that this CPU is going offline.
*/
static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
{
RCU_TRACE(unsigned long mask);
RCU_TRACE(struct rcu_data *rdp = this_cpu_ptr(rsp->rda));
RCU_TRACE(struct rcu_node *rnp = rdp->mynode);
RCU_TRACE(mask = rdp->grpmask);
trace_rcu_grace_period(rsp->name,
rnp->gpnum + 1 - !!(rnp->qsmask & mask),
"cpuofl");
rcu_report_qs_rdp(smp_processor_id(), rsp, rdp, rsp->gpnum);
/* Note that rcu_report_qs_rdp() might call trace_rcu_grace_period(). */
}
/*
* The CPU has been completely removed, and some other CPU is reporting
* this fact from process context. Do the remainder of the cleanup.
* this fact from process context. Do the remainder of the cleanup,
* including orphaning the outgoing CPU's RCU callbacks, and also
* adopting them, if there is no _rcu_barrier() instance running.
* There can only be one CPU hotplug operation at a time, so no other
* CPU can be attempting to update rcu_cpu_kthread_task.
*/
@ -1409,17 +1456,21 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
unsigned long mask;
int need_report = 0;
struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
struct rcu_node *rnp = rdp->mynode; /* Outgoing CPU's rnp. */
struct rcu_node *rnp = rdp->mynode; /* Outgoing CPU's rdp & rnp. */
/* Adjust any no-longer-needed kthreads. */
rcu_stop_cpu_kthread(cpu);
rcu_node_kthread_setaffinity(rnp, -1);
/* Remove the dying CPU from the bitmasks in the rcu_node hierarchy. */
/* Remove the dead CPU from the bitmasks in the rcu_node hierarchy. */
/* Exclude any attempts to start a new grace period. */
raw_spin_lock_irqsave(&rsp->onofflock, flags);
/* Orphan the dead CPU's callbacks, and adopt them if appropriate. */
rcu_send_cbs_to_orphanage(cpu, rsp, rnp, rdp);
rcu_adopt_orphan_cbs(rsp);
/* Remove the outgoing CPU from the masks in the rcu_node hierarchy. */
mask = rdp->grpmask; /* rnp->grplo is constant. */
do {
@ -1456,6 +1507,10 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
#else /* #ifdef CONFIG_HOTPLUG_CPU */
static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
{
}
static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
{
}
@ -1524,9 +1579,6 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
rcu_is_callbacks_kthread());
/* Update count, and requeue any remaining callbacks. */
rdp->qlen_lazy -= count_lazy;
rdp->qlen -= count;
rdp->n_cbs_invoked += count;
if (list != NULL) {
*tail = rdp->nxtlist;
rdp->nxtlist = list;
@ -1536,6 +1588,10 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
else
break;
}
smp_mb(); /* List handling before counting for rcu_barrier(). */
rdp->qlen_lazy -= count_lazy;
rdp->qlen -= count;
rdp->n_cbs_invoked += count;
/* Reinstate batch limit if we have worked down the excess. */
if (rdp->blimit == LONG_MAX && rdp->qlen <= qlowmark)
@ -1824,13 +1880,14 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
rdp = this_cpu_ptr(rsp->rda);
/* Add the callback to our list. */
*rdp->nxttail[RCU_NEXT_TAIL] = head;
rdp->nxttail[RCU_NEXT_TAIL] = &head->next;
rdp->qlen++;
if (lazy)
rdp->qlen_lazy++;
else
rcu_idle_count_callbacks_posted();
smp_mb(); /* Count before adding callback for rcu_barrier(). */
*rdp->nxttail[RCU_NEXT_TAIL] = head;
rdp->nxttail[RCU_NEXT_TAIL] = &head->next;
if (__is_kfree_rcu_offset((unsigned long)func))
trace_rcu_kfree_callback(rsp->name, head, (unsigned long)func,
@ -2169,11 +2226,10 @@ static int rcu_cpu_has_callbacks(int cpu)
rcu_preempt_cpu_has_callbacks(cpu);
}
static DEFINE_PER_CPU(struct rcu_head, rcu_barrier_head) = {NULL};
static atomic_t rcu_barrier_cpu_count;
static DEFINE_MUTEX(rcu_barrier_mutex);
static struct completion rcu_barrier_completion;
/*
* RCU callback function for _rcu_barrier(). If we are last, wake
* up the task executing _rcu_barrier().
*/
static void rcu_barrier_callback(struct rcu_head *notused)
{
if (atomic_dec_and_test(&rcu_barrier_cpu_count))
@ -2203,27 +2259,94 @@ static void _rcu_barrier(struct rcu_state *rsp,
void (*call_rcu_func)(struct rcu_head *head,
void (*func)(struct rcu_head *head)))
{
BUG_ON(in_interrupt());
int cpu;
unsigned long flags;
struct rcu_data *rdp;
struct rcu_head rh;
init_rcu_head_on_stack(&rh);
/* Take mutex to serialize concurrent rcu_barrier() requests. */
mutex_lock(&rcu_barrier_mutex);
init_completion(&rcu_barrier_completion);
smp_mb(); /* Prevent any prior operations from leaking in. */
/*
* Initialize rcu_barrier_cpu_count to 1, then invoke
* rcu_barrier_func() on each CPU, so that each CPU also has
* incremented rcu_barrier_cpu_count. Only then is it safe to
* decrement rcu_barrier_cpu_count -- otherwise the first CPU
* might complete its grace period before all of the other CPUs
* did their increment, causing this function to return too
* early. Note that on_each_cpu() disables irqs, which prevents
* any CPUs from coming online or going offline until each online
* CPU has queued its RCU-barrier callback.
* Initialize the count to one rather than to zero in order to
* avoid a too-soon return to zero in case of a short grace period
* (or preemption of this task). Also flag this task as doing
* an rcu_barrier(). This will prevent anyone else from adopting
* orphaned callbacks, which could cause otherwise failure if a
* CPU went offline and quickly came back online. To see this,
* consider the following sequence of events:
*
* 1. We cause CPU 0 to post an rcu_barrier_callback() callback.
* 2. CPU 1 goes offline, orphaning its callbacks.
* 3. CPU 0 adopts CPU 1's orphaned callbacks.
* 4. CPU 1 comes back online.
* 5. We cause CPU 1 to post an rcu_barrier_callback() callback.
* 6. Both rcu_barrier_callback() callbacks are invoked, awakening
* us -- but before CPU 1's orphaned callbacks are invoked!!!
*/
init_completion(&rcu_barrier_completion);
atomic_set(&rcu_barrier_cpu_count, 1);
on_each_cpu(rcu_barrier_func, (void *)call_rcu_func, 1);
raw_spin_lock_irqsave(&rsp->onofflock, flags);
rsp->rcu_barrier_in_progress = current;
raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
/*
* Force every CPU with callbacks to register a new callback
* that will tell us when all the preceding callbacks have
* been invoked. If an offline CPU has callbacks, wait for
* it to either come back online or to finish orphaning those
* callbacks.
*/
for_each_possible_cpu(cpu) {
preempt_disable();
rdp = per_cpu_ptr(rsp->rda, cpu);
if (cpu_is_offline(cpu)) {
preempt_enable();
while (cpu_is_offline(cpu) && ACCESS_ONCE(rdp->qlen))
schedule_timeout_interruptible(1);
} else if (ACCESS_ONCE(rdp->qlen)) {
smp_call_function_single(cpu, rcu_barrier_func,
(void *)call_rcu_func, 1);
preempt_enable();
} else {
preempt_enable();
}
}
/*
* Now that all online CPUs have rcu_barrier_callback() callbacks
* posted, we can adopt all of the orphaned callbacks and place
* an rcu_barrier_callback() callback after them. When that is done,
* we are guaranteed to have an rcu_barrier_callback() callback
* following every callback that could possibly have been
* registered before _rcu_barrier() was called.
*/
raw_spin_lock_irqsave(&rsp->onofflock, flags);
rcu_adopt_orphan_cbs(rsp);
rsp->rcu_barrier_in_progress = NULL;
raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
atomic_inc(&rcu_barrier_cpu_count);
smp_mb__after_atomic_inc(); /* Ensure atomic_inc() before callback. */
call_rcu_func(&rh, rcu_barrier_callback);
/*
* Now that we have an rcu_barrier_callback() callback on each
* CPU, and thus each counted, remove the initial count.
*/
if (atomic_dec_and_test(&rcu_barrier_cpu_count))
complete(&rcu_barrier_completion);
/* Wait for all rcu_barrier_callback() callbacks to be invoked. */
wait_for_completion(&rcu_barrier_completion);
/* Other rcu_barrier() invocations can now safely proceed. */
mutex_unlock(&rcu_barrier_mutex);
destroy_rcu_head_on_stack(&rh);
}
/**

11
kernel/rcutree.h
View file

@ -371,6 +371,17 @@ struct rcu_state {
raw_spinlock_t onofflock; /* exclude on/offline and */
/* starting new GP. */
struct rcu_head *orphan_nxtlist; /* Orphaned callbacks that */
/* need a grace period. */
struct rcu_head **orphan_nxttail; /* Tail of above. */
struct rcu_head *orphan_donelist; /* Orphaned callbacks that */
/* are ready to invoke. */
struct rcu_head **orphan_donetail; /* Tail of above. */
long qlen_lazy; /* Number of lazy callbacks. */
long qlen; /* Total number of callbacks. */
struct task_struct *rcu_barrier_in_progress;
/* Task doing rcu_barrier(), */
/* or NULL if no barrier. */
raw_spinlock_t fqslock; /* Only one task forcing */
/* quiescent states. */
unsigned long jiffies_force_qs; /* Time at which to invoke */

4
kernel/rcutree_trace.c
View file

@ -271,13 +271,13 @@ static void print_one_rcu_state(struct seq_file *m, struct rcu_state *rsp)
gpnum = rsp->gpnum;
seq_printf(m, "c=%lu g=%lu s=%d jfq=%ld j=%x "
"nfqs=%lu/nfqsng=%lu(%lu) fqlh=%lu\n",
"nfqs=%lu/nfqsng=%lu(%lu) fqlh=%lu oqlen=%ld/%ld\n",
rsp->completed, gpnum, rsp->fqs_state,
(long)(rsp->jiffies_force_qs - jiffies),
(int)(jiffies & 0xffff),
rsp->n_force_qs, rsp->n_force_qs_ngp,
rsp->n_force_qs - rsp->n_force_qs_ngp,
rsp->n_force_qs_lh);
rsp->n_force_qs_lh, rsp->qlen_lazy, rsp->qlen);
for (rnp = &rsp->node[0]; rnp - &rsp->node[0] < NUM_RCU_NODES; rnp++) {
if (rnp->level != level) {
seq_puts(m, "\n");