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Commit e52fb7c0 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
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sched: prefer wakers 



Prefer tasks that wake other tasks to preempt quickly. This improves
performance because more work is available sooner.

The workload that prompted this patch was a kernel build over NFS4 (for some
curious and not understood reason we had to revert commit:
18de9735 to make any progress at all)

Without this patch a make -j8 bzImage (of x86-64 defconfig) would take
3m30-ish, with this patch we're down to 2m50-ish.

psql-sysbench/mysql-sysbench show a slight improvement in peak performance as
well, tbench and vmark seemed to not care.

It is possible to improve upon the build time (to 2m20-ish) but that seriously
destroys other benchmarks (just shows that there's more room for tinkering).

Much thanks to Mike who put in a lot of effort to benchmark things and proved
a worthy opponent with a competing patch.

Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: default avatarMike Galbraith <efault@gmx.de>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 831451ac

kernel/sched_fair.c

+53 −6
Original line number Diff line number Diff line
@@ -1295,16 +1295,63 @@ static int select_task_rq_fair(struct task_struct *p, int sync) 
}

#endif /* CONFIG_SMP */



static unsigned long wakeup_gran(struct sched_entity *se)

/*

 * Adaptive granularity

 *

 * se->avg_wakeup gives the average time a task runs until it does a wakeup,

 * with the limit of wakeup_gran -- when it never does a wakeup.

 *

 * So the smaller avg_wakeup is the faster we want this task to preempt,

 * but we don't want to treat the preemptee unfairly and therefore allow it

 * to run for at least the amount of time we'd like to run.

 *

 * NOTE: we use 2*avg_wakeup to increase the probability of actually doing one

 *

 * NOTE: we use *nr_running to scale with load, this nicely matches the

 *       degrading latency on load.

 */

static unsigned long

adaptive_gran(struct sched_entity *curr, struct sched_entity *se)

{

	u64 this_run = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;

	u64 expected_wakeup = 2*se->avg_wakeup * cfs_rq_of(se)->nr_running;

	u64 gran = 0;



	if (this_run < expected_wakeup)

		gran = expected_wakeup - this_run;



	return min_t(s64, gran, sysctl_sched_wakeup_granularity);

}



static unsigned long

wakeup_gran(struct sched_entity *curr, struct sched_entity *se)

{

	unsigned long gran = sysctl_sched_wakeup_granularity;



	if (cfs_rq_of(curr)->curr && sched_feat(ADAPTIVE_GRAN))

		gran = adaptive_gran(curr, se);



	/*

	 * Since its curr running now, convert the gran from real-time

	 * to virtual-time in his units.

	 */

	if (sched_feat(ASYM_GRAN)) {

		/*

	 * More easily preempt - nice tasks, while not making it harder for

	 * + nice tasks.

		 * By using 'se' instead of 'curr' we penalize light tasks, so

		 * they get preempted easier. That is, if 'se' < 'curr' then

		 * the resulting gran will be larger, therefore penalizing the

		 * lighter, if otoh 'se' > 'curr' then the resulting gran will

		 * be smaller, again penalizing the lighter task.

		 *

		 * This is especially important for buddies when the leftmost

		 * task is higher priority than the buddy.

		 */

	if (!sched_feat(ASYM_GRAN) || se->load.weight > NICE_0_LOAD)

		gran = calc_delta_fair(sysctl_sched_wakeup_granularity, se);

		if (unlikely(se->load.weight != NICE_0_LOAD))

			gran = calc_delta_fair(gran, se);

	} else {

		if (unlikely(curr->load.weight != NICE_0_LOAD))

			gran = calc_delta_fair(gran, curr);

	}



	return gran;

}
@@ -1331,7 +1378,7 @@ wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se) 
	if (vdiff <= 0)

		return -1;



	gran = wakeup_gran(curr);

	gran = wakeup_gran(curr, se);

	if (vdiff > gran)

		return 1;

kernel/sched_features.h

+2 −1
Original line number Diff line number Diff line 
SCHED_FEAT(NEW_FAIR_SLEEPERS, 1)

SCHED_FEAT(NORMALIZED_SLEEPER, 1)

SCHED_FEAT(NORMALIZED_SLEEPER, 0)

SCHED_FEAT(ADAPTIVE_GRAN, 1)

SCHED_FEAT(WAKEUP_PREEMPT, 1)

SCHED_FEAT(START_DEBIT, 1)

SCHED_FEAT(AFFINE_WAKEUPS, 1)