linux-kernel - Re: [PATCH 0/1] RFC: sched/fair: skip select_idle

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Date:   Wed, 9 Jan 2019 13:24:18 -0500
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Mike Galbraith <efault@....de>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/1] RFC: sched/fair: skip select_idle_sibling() in
 presence of sync wakeups

On Wed, Jan 09, 2019 at 10:07:51AM +0000, Mel Gorman wrote:
> I agree with Mike here. Many previous attempts to strictly obey the strict
> hint has led to regressions elsewhere -- specifically a task waking 2+
> wakees that temporarily stack on one CPU when nearby CPUs sharing LLC

sync-waking 2 wakees in a row before going to sleep should cause the
runqueue to have nr_running != 1 after the first wakee is waken up on
the local CPU. Your example explains the following nr_running == 1
check in the patch:

	    (sync && target == this_cpu && cpu_rq(this_cpu)->nr_running == 1))
					   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The implementation is just an RFC because it may have other drawbacks,
but I thought the second wakee of this specific example supposedly
should still do the idle core/ht balancing normally like before the
change.

Thanks,
Andrea