Messages in this thread Patch in this message | | | Date | Mon, 12 Feb 2018 18:01:43 +0000 | From | Mel Gorman <> | Subject | Re: [PATCH 4/4] sched/fair: Do not migrate due to a sync wakeup on exit |
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On Mon, Feb 12, 2018 at 05:47:29PM +0000, Mel Gorman wrote: > On Mon, Feb 12, 2018 at 06:31:47PM +0100, Peter Zijlstra wrote: > > On Mon, Feb 12, 2018 at 02:58:57PM +0000, Mel Gorman wrote: > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > > index 28c8d9c91955..50442697b455 100644 > > > --- a/kernel/sched/fair.c > > > +++ b/kernel/sched/fair.c > > > @@ -5710,8 +5710,14 @@ wake_affine_idle(int this_cpu, int prev_cpu, int sync) > > > if (idle_cpu(this_cpu) && cpus_share_cache(this_cpu, prev_cpu)) > > > return idle_cpu(prev_cpu) ? prev_cpu : this_cpu; > > > > > > - if (sync && cpu_rq(this_cpu)->nr_running == 1) > > > + if (sync && cpu_rq(this_cpu)->nr_running == 1) { > > > + /* Avoid tasks exiting pulling parents to new nodes */ > > > + if ((current->flags & PF_EXITING) && > > > + !cpus_share_cache(this_cpu, prev_cpu)) > > > + return prev_cpu; > > > + > > > > Cute, but should we not kill @sync right at the source in this case? > > > > Something a little like this? > > > > That works too but note that it has a slightly disadvantage in that we > add weight to the select_task_rq_fair fast path for all wakeups, not just > those that are affine. It's also not 100% functionally equivalent as I was > still allowing parents to migrate to the CPU if it shared cache although I > expect any advantage there is marginal. I can replace my patch with yours > if you really prefer it as the overhead in the fast path should be marginal. >
Clearly my brains leaked out my ears at the end of a work day. The overhead only applies to sync wakeups and is a very small amount of overhead in the case where this_cpu or prev_cpu was idle so I'm happy with either version.
-- Mel Gorman SUSE Labs
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