linux-kernel - Re: [PATCH RESEND] sched: prefer an idle cpu vs an idle sibling for BALANCE

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Date:	Sat, 30 May 2015 05:55:49 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Josef Bacik <jbacik@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>, riel@...hat.com,
	mingo@...hat.com, linux-kernel@...r.kernel.org,
	morten.rasmussen@....com, kernel-team <Kernel-team@...com>
Subject: Re: [PATCH RESEND] sched: prefer an idle cpu vs an idle sibling for
 BALANCE_WAKE

On Fri, 2015-05-29 at 17:03 -0400, Josef Bacik wrote:

> for_each_lower_domain(sd) {
>          sg = sd->groups;
>          do {
>                  if (!cpumask_intersects(sched_group_cpus(sg),
>                                          tsk_cpus_allowed(p)))
>                          goto next;
> 
>                  for_each_cpu(i, sched_group_cpus(sg)) {
>                          if (i == target || !idle_cpu(i))
>                                  goto next;
>                  }
> 
>                  return cpumask_first_and(sched_group_cpus(sg),
>                                  tsk_cpus_allowed(p));
> next:
>                  sg = sg->next
>          } while (sg != sd->groups);
> }
> 
> We get all the schedule groups for the schedule domain and if any of the 
> cpu's are not idle or the target then we skip the whole scheduling 
> group.  Isn't the scheduling group a group of CPU's?  Why can't we pick 
> an idle CPU in the group that has a none idle cpu or the target cpu? 
> Thanks,

We select an idle core if we can get one.  Yes, that leaves a pile of
SMT threads not checked/selected, but if you're gonna do a full search
of a large socket (humongous sparc-thing, shudder), you may as well eat
the BALANCE_WAKE overhead.

	-Mike

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