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sched/core: Create preempt_count invariant

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Assuming units of PREEMPT_DISABLE_OFFSET for preempt_count() numbers.

Now that TASK_DEAD no longer results in preempt_count() == 3 during
scheduling, we will always call context_switch() with preempt_count()
== 2.

However, we don't always end up with preempt_count() == 2 in
finish_task_switch() because new tasks get created with
preempt_count() == 1.

Create FORK_PREEMPT_COUNT and set it to 2 and use that in the right
places. Note that we cannot use INIT_PREEMPT_COUNT as that serves
another purpose (boot).

After this, preempt_count() is invariant across the context switch,
with exception of PREEMPT_ACTIVE.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2015-09-28 17:52:18 +02:00 committed by Ingo Molnar
parent b99def8b96
commit 609ca06638
4 changed files with 35 additions and 9 deletions
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2
arch/x86/include/asm/preempt.h
View file

@ -31,7 +31,7 @@ static __always_inline void preempt_count_set(int pc)
* must be macros to avoid header recursion hell
*/
#define init_task_preempt_count(p) do { \
task_thread_info(p)->saved_preempt_count = PREEMPT_DISABLED; \
task_thread_info(p)->saved_preempt_count = FORK_PREEMPT_COUNT; \
} while (0)
#define init_idle_preempt_count(p, cpu) do { \

2
include/asm-generic/preempt.h
View file

@ -24,7 +24,7 @@ static __always_inline void preempt_count_set(int pc)
* must be macros to avoid header recursion hell
*/
#define init_task_preempt_count(p) do { \
task_thread_info(p)->preempt_count = PREEMPT_DISABLED; \
task_thread_info(p)->preempt_count = FORK_PREEMPT_COUNT; \
} while (0)
#define init_idle_preempt_count(p, cpu) do { \

17
include/linux/sched.h
View file

@ -599,11 +599,7 @@ struct task_cputime_atomic {
.sum_exec_runtime = ATOMIC64_INIT(0), \
}
#ifdef CONFIG_PREEMPT_COUNT
#define PREEMPT_DISABLED (1 + PREEMPT_ENABLED)
#else
#define PREEMPT_DISABLED PREEMPT_ENABLED
#endif
#define PREEMPT_DISABLED (PREEMPT_DISABLE_OFFSET + PREEMPT_ENABLED)
/*
* Disable preemption until the scheduler is running -- use an unconditional
@ -613,6 +609,17 @@ struct task_cputime_atomic {
*/
#define INIT_PREEMPT_COUNT PREEMPT_OFFSET
/*
* Initial preempt_count value; reflects the preempt_count schedule invariant
* which states that during context switches:
*
* preempt_count() == 2*PREEMPT_DISABLE_OFFSET
*
* Note: PREEMPT_DISABLE_OFFSET is 0 for !PREEMPT_COUNT kernels.
* Note: See finish_task_switch().
*/
#define FORK_PREEMPT_COUNT (2*PREEMPT_DISABLE_OFFSET + PREEMPT_ENABLED)
/**
* struct thread_group_cputimer - thread group interval timer counts
* @cputime_atomic: atomic thread group interval timers.

23
kernel/sched/core.c
View file

@ -2504,6 +2504,18 @@ static struct rq *finish_task_switch(struct task_struct *prev)
struct mm_struct *mm = rq->prev_mm;
long prev_state;
/*
* The previous task will have left us with a preempt_count of 2
* because it left us after:
*
* schedule()
* preempt_disable(); // 1
* __schedule()
* raw_spin_lock_irq(&rq->lock) // 2
*
* Also, see FORK_PREEMPT_COUNT.
*/
rq->prev_mm = NULL;
/*
@ -2588,8 +2600,15 @@ asmlinkage __visible void schedule_tail(struct task_struct *prev)
{
struct rq *rq;
/* finish_task_switch() drops rq->lock and enables preemtion */
preempt_disable();
/*
* New tasks start with FORK_PREEMPT_COUNT, see there and
* finish_task_switch() for details.
*
* finish_task_switch() will drop rq->lock() and lower preempt_count
* and the preempt_enable() will end up enabling preemption (on
* PREEMPT_COUNT kernels).
*/
rq = finish_task_switch(prev);
balance_callback(rq);
preempt_enable();