On Wed, 2013-09-25 at 09:53 +0200, Peter Zijlstra wrote: > Subject: sched: Avoid select_idle_sibling() for wake_affine(.sync=true) > From: Peter Zijlstra <[email protected]> > Date: Wed Sep 25 08:28:39 CEST 2013 > > When a task is the only running task and does a sync wakeup; avoid > going through select_idle_sibling() as it doesn't know the current CPU > is going to be idle shortly. > > Without this two sync wakers will ping-pong between CPUs for no > reason.
That will make pipe-test go fugly -> pretty, and help very fast/light localhost network, but eat heavier localhost overlap recovery. We need a working (and cheap) overlap detector scheme, so we can know when there is enough to be worth going after. (I sent you some lmbench numbers offline a while back showing the two-faced little <b-word> in action, doing both good and evil) > Suggested-by: Rik van Riel <[email protected]> > Signed-off-by: Peter Zijlstra <[email protected]> > --- > kernel/sched/fair.c | 10 ++++++++++ > 1 file changed, 10 insertions(+) > > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -3461,6 +3461,16 @@ select_task_rq_fair(struct task_struct * > if (cpu != prev_cpu && wake_affine(affine_sd, p, sync)) > prev_cpu = cpu; > > + /* > + * Don't bother with select_idle_sibling() in the case of a > sync wakeup > + * where we know the only running task will soon go away. Going > + * through select_idle_sibling will only lead to pointless > ping-pong. > + */ > + if (sync && prev_cpu == cpu && cpu_rq(cpu)->nr_running == 1) { > + new_cpu = cpu; > + goto unlock; > + } > + > new_cpu = select_idle_sibling(p, prev_cpu); > goto unlock; > } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
