Commit f5bfb7d9 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

sched: bias effective_load() error towards failing wake_affine(). 



Measurement shows that the difference between cgroup:/ and cgroup:/foo
wake_affine() results is that the latter succeeds significantly more.

Therefore bias the calculations towards failing the test.

Signed-off-by: default avatarPeter Zijlstra <peterz@infradead.org>
Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent f1d239f7

kernel/sched_fair.c

+28 −0
Original line number Diff line number Diff line
@@ -1074,6 +1074,27 @@ static inline int wake_idle(int cpu, struct task_struct *p) 
static const struct sched_class fair_sched_class;



#ifdef CONFIG_FAIR_GROUP_SCHED

/*

 * effective_load() calculates the load change as seen from the root_task_group

 *

 * Adding load to a group doesn't make a group heavier, but can cause movement

 * of group shares between cpus. Assuming the shares were perfectly aligned one

 * can calculate the shift in shares.

 *

 * The problem is that perfectly aligning the shares is rather expensive, hence

 * we try to avoid doing that too often - see update_shares(), which ratelimits

 * this change.

 *

 * We compensate this by not only taking the current delta into account, but

 * also considering the delta between when the shares were last adjusted and

 * now.

 *

 * We still saw a performance dip, some tracing learned us that between

 * cgroup:/ and cgroup:/foo balancing the number of affine wakeups increased

 * significantly. Therefore try to bias the error in direction of failing

 * the affine wakeup.

 *

 */

static long effective_load(struct task_group *tg, int cpu,

		long wl, long wg)

{
@@ -1083,6 +1104,13 @@ static long effective_load(struct task_group *tg, int cpu, 
	if (!tg->parent)

		return wl;



	/*

	 * By not taking the decrease of shares on the other cpu into

	 * account our error leans towards reducing the affine wakeups.

	 */

	if (!wl && sched_feat(ASYM_EFF_LOAD))

		return wl;



	/*

	 * Instead of using this increment, also add the difference

	 * between when the shares were last updated and now.

kernel/sched_features.h

+1 −0
Original line number Diff line number Diff line
@@ -10,3 +10,4 @@ SCHED_FEAT(DOUBLE_TICK, 0) 
SCHED_FEAT(ASYM_GRAN, 1)

SCHED_FEAT(LB_BIAS, 0)

SCHED_FEAT(LB_WAKEUP_UPDATE, 1)

SCHED_FEAT(ASYM_EFF_LOAD, 1)

CRA Git | Maintained and supported by SUSTech CRA and CCSE