Commit 05b40e05 authored Mar 22, 2017 by Srikar Dronamraju Committed by Ingo Molnar Mar 27, 2017

sched/fair: Prefer sibiling only if local group is under-utilized

If the child domain prefers tasks to go siblings, the local group could
end up pulling tasks to itself even if the local group is almost equally
loaded as the source group.

Lets assume a 4 core,smt==2 machine running 5 thread ebizzy workload.
Everytime, local group has capacity and source group has atleast 2 threads,
local group tries to pull the task. This causes the threads to constantly
move between different cores. This is even more profound if the cores have
more threads, like in Power 8, smt 8 mode.

Fix this by only allowing local group to pull a task, if the source group
has more number of tasks than the local group.

Here are the relevant perf stat numbers of a 22 core,smt 8 Power 8 machine.

Without patch:
Performance counter stats for 'ebizzy -t 22 -S 100' (5 runs):

1,440 context-switches # 0.001 K/sec ( +- 1.26% )
366 cpu-migrations # 0.000 K/sec ( +- 5.58% )
3,933 page-faults # 0.002 K/sec ( +- 11.08% )

Performance counter stats for 'ebizzy -t 48 -S 100' (5 runs):

6,287 context-switches # 0.001 K/sec ( +- 3.65% )
3,776 cpu-migrations # 0.001 K/sec ( +- 4.84% )
5,702 page-faults # 0.001 K/sec ( +- 9.36% )

Performance counter stats for 'ebizzy -t 96 -S 100' (5 runs):

8,776 context-switches # 0.001 K/sec ( +- 0.73% )
2,790 cpu-migrations # 0.000 K/sec ( +- 0.98% )
10,540 page-faults # 0.001 K/sec ( +- 3.12% )

With patch:

Performance counter stats for 'ebizzy -t 22 -S 100' (5 runs):

1,133 context-switches # 0.001 K/sec ( +- 4.72% )
123 cpu-migrations # 0.000 K/sec ( +- 3.42% )
3,858 page-faults # 0.002 K/sec ( +- 8.52% )

Performance counter stats for 'ebizzy -t 48 -S 100' (5 runs):

2,169 context-switches # 0.000 K/sec ( +- 6.19% )
189 cpu-migrations # 0.000 K/sec ( +- 12.75% )
5,917 page-faults # 0.001 K/sec ( +- 8.09% )

Performance counter stats for 'ebizzy -t 96 -S 100' (5 runs):

5,333 context-switches # 0.001 K/sec ( +- 5.91% )
506 cpu-migrations # 0.000 K/sec ( +- 3.35% )
10,792 page-faults # 0.001 K/sec ( +- 7.75% )

Which show that in these workloads CPU migrations get reduced significantly.

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
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: Vincent Guittot <vincent.guittot@linaro.org>
Link: http://lkml.kernel.org/r/1490205470-10249-1-git-send-email-srikar@linux.vnet.ibm.com

Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent bc427898

kernel/sched/fair.c

+4 −3

Original line number	Diff line number	Diff line
		@@ -7565,6 +7565,7 @@ static inline void update_sd_lb_stats(struct lb_env env, struct sd_lb_stats sd
		{
		struct sched_domain *child = env->sd->child;
		struct sched_group *sg = env->sd->groups;
		struct sg_lb_stats *local = &sds->local_stat;
		struct sg_lb_stats tmp_sgs;
		int load_idx, prefer_sibling = 0;
		bool overload = false;
		@@ -7581,7 +7582,7 @@ static inline void update_sd_lb_stats(struct lb_env env, struct sd_lb_stats sd
		local_group = cpumask_test_cpu(env->dst_cpu, sched_group_cpus(sg));
		if (local_group) {
		sds->local = sg;
		sgs = &sds->local_stat;
		sgs = local;

		if (env->idle != CPU_NEWLY_IDLE \|\|
		time_after_eq(jiffies, sg->sgc->next_update))
		@@ -7605,8 +7606,8 @@ static inline void update_sd_lb_stats(struct lb_env env, struct sd_lb_stats sd
		* the tasks on the system).
		*/
		if (prefer_sibling && sds->local &&
		group_has_capacity(env, &sds->local_stat) &&
		(sgs->sum_nr_running > 1)) {
		group_has_capacity(env, local) &&
		(sgs->sum_nr_running > local->sum_nr_running + 1)) {
		sgs->group_no_capacity = 1;
		sgs->group_type = group_classify(sg, sgs);
		}

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