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sched/core: Rework rq->clock update skips

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The original purpose of rq::skip_clock_update was to avoid 'costly' clock
updates for back to back wakeup-preempt pairs. The big problem with it
has always been that the rq variable is unaware of the context and
causes indiscrimiate clock skips.

Rework the entire thing and create a sense of context by only allowing
schedule() to skip clock updates. (XXX can we measure the cost of the
added store?)

By ensuring only schedule can ever skip an update, we guarantee we're
never more than 1 tick behind on the update.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: umgwanakikbuti@gmail.com
Link: http://lkml.kernel.org/r/20150105103554.432381549@infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra
2015-01-05 11:18:11 +01:00
committed by Ingo Molnar
parent cebde6d681
commit 9edfbfed3f
4 changed files with 28 additions and 10 deletions
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12
kernel/sched/core.c
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@@ -119,7 +119,9 @@ void update_rq_clock(struct rq *rq)
{
s64 delta;
if (rq->skip_clock_update > 0)
lockdep_assert_held(&rq->lock);
if (rq->clock_skip_update & RQCF_ACT_SKIP)
return;
delta = sched_clock_cpu(cpu_of(rq)) - rq->clock;
@@ -1046,7 +1048,7 @@ void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags)
* this case, we can save a useless back to back clock update.
*/
if (task_on_rq_queued(rq->curr) && test_tsk_need_resched(rq->curr))
rq->skip_clock_update = 1;
rq_clock_skip_update(rq, true);
}
#ifdef CONFIG_SMP
@@ -2779,6 +2781,8 @@ need_resched:
smp_mb__before_spinlock();
raw_spin_lock_irq(&rq->lock);
rq->clock_skip_update <<= 1; /* promote REQ to ACT */
switch_count = &prev->nivcsw;
if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
if (unlikely(signal_pending_state(prev->state, prev))) {
@@ -2803,13 +2807,13 @@ need_resched:
switch_count = &prev->nvcsw;
}
if (task_on_rq_queued(prev) || rq->skip_clock_update < 0)
if (task_on_rq_queued(prev))
update_rq_clock(rq);
next = pick_next_task(rq, prev);
clear_tsk_need_resched(prev);
clear_preempt_need_resched();
rq->skip_clock_update = 0;
rq->clock_skip_update = 0;
if (likely(prev != next)) {
rq->nr_switches++;