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sched: Add NEED_RESCHED to the preempt_count

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In order to combine the preemption and need_resched test we need to
fold the need_resched information into the preempt_count value.

Since the NEED_RESCHED flag is set across CPUs this needs to be an
atomic operation, however we very much want to avoid making
preempt_count atomic, therefore we keep the existing TIF_NEED_RESCHED
infrastructure in place but at 3 sites test it and fold its value into
preempt_count; namely:

 - resched_task() when setting TIF_NEED_RESCHED on the current task
 - scheduler_ipi() when resched_task() sets TIF_NEED_RESCHED on a
                   remote task it follows it up with a reschedule IPI
                   and we can modify the cpu local preempt_count from
                   there.
 - cpu_idle_loop() for when resched_task() found tsk_is_polling().

We use an inverted bitmask to indicate need_resched so that a 0 means
both need_resched and !atomic.

Also remove the barrier() in preempt_enable() between
preempt_enable_no_resched() and preempt_check_resched() to avoid
having to reload the preemption value and allow the compiler to use
the flags of the previuos decrement. I couldn't come up with any sane
reason for this barrier() to be there as preempt_enable_no_resched()
already has a barrier() before doing the decrement.

Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-7a7m5qqbn5pmwnd4wko9u6da@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2013-08-14 14:55:31 +02:00 • committed by Ingo Molnar
commit f27dde8dee
4 changed files with 69 additions and 12 deletions
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7
include/linux/sched.h
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@ -22,6 +22,7 @@ struct sched_param {
#include <linux/errno.h>
#include <linux/nodemask.h>
#include <linux/mm_types.h>
#include <linux/preempt.h>
#include <asm/page.h>
#include <asm/ptrace.h>
@ -434,7 +435,9 @@ struct task_cputime {
* We include PREEMPT_ACTIVE to avoid cond_resched() from working
* before the scheduler is active -- see should_resched().
*/
#define INIT_PREEMPT_COUNT (1 + PREEMPT_ACTIVE)
#define INIT_PREEMPT_COUNT (1 + PREEMPT_ACTIVE + PREEMPT_NEED_RESCHED)
#define PREEMPT_ENABLED (PREEMPT_NEED_RESCHED)
#define PREEMPT_DISABLED (1 + PREEMPT_NEED_RESCHED)
/**
* struct thread_group_cputimer - thread group interval timer counts
@ -2408,7 +2411,7 @@ static inline int signal_pending_state(long state, struct task_struct *p)
static inline int need_resched(void)
{
return unlikely(test_thread_flag(TIF_NEED_RESCHED));
return unlikely(test_preempt_need_resched());
}
/*