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Date:   Thu, 31 Oct 2019 10:19:04 +0000
From:   Mel Gorman <mgorman@suse.de>
To:     Viresh Kumar <viresh.kumar@linaro.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] sched/fair: Make sched-idle cpu selection consistent
 throughout
Message-ID: <20191031101904.GI28938@suse.de>
References: <5eba2fb4af9ebc7396101bb9bd6c8aa9c8af0710.1571899508.git.viresh.kumar@linaro.org>
 <20191030164714.GH28938@suse.de>
 <CAKohpo=hssu_uvb1J=0Od=KziAQVSMmbBt9zxa4mYttKhFJwFw@mail.gmail.com>
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On Thu, Oct 31, 2019 at 02:42:03PM +0530, Viresh Kumar wrote:
> On Wed, 30 Oct 2019 at 22:17, Mel Gorman <mgorman@suse.de> wrote:
> 
> > As the patch stands, I think a fork-intensive workload where each
> > process is doing small amounts of work will suffer from overloading
> > domains and have variable performance depending on how quickly the load
> > balancer reacts.
> 
> Just wanted to clarify this slightly in case it is confusing. Once a
> newly forked
> (non SCHED_IDLE) task gets placed on a sched-idle CPU, it won't remain
> sched-idle anymore and we will again start looking for a fully idle CPU. So,
> we won't put everything on a small set of CPUs, but just one SCHED_NORMAL
> task on a CPU unless we are out of idle CPUs.
> 
> Do you have some specific test in mind which I can run to test this ?
> 

Nothing in particular. git test suite for the basic fork-intensive case
(mmtests config workload-shellscripts), something fork-intensive but
relatively short-lived like a kernel build scaling the number of build
jobs (mmtests config config-workload-kerndevel), something fairly basic
that scales number of running jobs and relatively long-lived like tbench
(mmtests config config-network-tbench). The ideal of course is that you
wrote the patch based on an observed problem that you decided to fix.

-- 
Mel Gorman
SUSE Labs