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Commit d4335581 authored by Matt Fleming's avatar Matt Fleming Committed by Ingo Molnar
Browse files

sched/fair: Add comments to explain select_idle_sibling() 



It's not entirely obvious how the main loop in select_idle_sibling()
works on first glance. Sprinkle a few comments to explain the design
and intention behind the loop based on some conversations with Mike
and Peter.

Signed-off-by: default avatarMatt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.com>
Cc: Mike Galbraith <mgalbraith@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1457535548-15329-1-git-send-email-matt@codeblueprint.co.uk


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 3a47d512

kernel/sched/fair.c

+18 −1
Original line number Diff line number Diff line
@@ -5055,7 +5055,19 @@ static int select_idle_sibling(struct task_struct *p, int target) 
		return i;



	/*

	 * Otherwise, iterate the domains and find an elegible idle cpu.

	 * Otherwise, iterate the domains and find an eligible idle cpu.

	 *

	 * A completely idle sched group at higher domains is more

	 * desirable than an idle group at a lower level, because lower

	 * domains have smaller groups and usually share hardware

	 * resources which causes tasks to contend on them, e.g. x86

	 * hyperthread siblings in the lowest domain (SMT) can contend

	 * on the shared cpu pipeline.

	 *

	 * However, while we prefer idle groups at higher domains

	 * finding an idle cpu at the lowest domain is still better than

	 * returning 'target', which we've already established, isn't

	 * idle.

	 */

	sd = rcu_dereference(per_cpu(sd_llc, target));

	for_each_lower_domain(sd) {
@@ -5065,11 +5077,16 @@ static int select_idle_sibling(struct task_struct *p, int target) 
						tsk_cpus_allowed(p)))

				goto next;



			/* Ensure the entire group is idle */

			for_each_cpu(i, sched_group_cpus(sg)) {

				if (i == target || !idle_cpu(i))

					goto next;

			}



			/*

			 * It doesn't matter which cpu we pick, the

			 * whole group is idle.

			 */

			target = cpumask_first_and(sched_group_cpus(sg),

					tsk_cpus_allowed(p));

			goto done;