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

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Date:   Wed, 9 Jan 2019 10:07:51 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Mike Galbraith <efault@....de>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        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 05:19:48AM +0100, Mike Galbraith wrote:
> > To test I used this trivial program.
> 
> Which highlights the problem.  That proggy really is synchronous, but
> the sync hint is applied to many MANY real world cases where this is
> not the case at all.  Sure, you can make things like pipe_test and
> nearly nonexistent payload TCP_RR numbers look gorgeous, but that
> demolishes concurrency for real applications.
> 

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
remain idle. It's why the select idle sibling logic tried to take into
account a recently used CPU to wake such tasks if the recent CPU was
still idle.

-- 
Mel Gorman
SUSE Labs