Commit-ID: d4335581dc30ec6545999c7443bb9fead274a980 Gitweb: http://git.kernel.org/tip/d4335581dc30ec6545999c7443bb9fead274a980 Author: Matt Fleming <m...@codeblueprint.co.uk> AuthorDate: Wed, 9 Mar 2016 14:59:08 +0000 Committer: Ingo Molnar <mi...@kernel.org> CommitDate: Mon, 21 Mar 2016 10:52:51 +0100
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: Matt Fleming <m...@codeblueprint.co.uk> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> Cc: Linus Torvalds <torva...@linux-foundation.org> Cc: Mel Gorman <mgor...@suse.com> Cc: Mike Galbraith <mgalbra...@suse.de> Cc: Peter Zijlstra <pet...@infradead.org> Cc: Thomas Gleixner <t...@linutronix.de> Link: http://lkml.kernel.org/r/1457535548-15329-1-git-send-email-m...@codeblueprint.co.uk Signed-off-by: Ingo Molnar <mi...@kernel.org> --- kernel/sched/fair.c | 19 ++++++++++++++++++- 1 file changed, 18 insertions(+), 1 deletion(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 3c114d9..303d639 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -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;
