linux-kernel - Re: [PATCH] sched: fix incorrect PELT values on SMT

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Message-ID: <20160819050313.GD11114@graphite.smuckle.net>
Date:   Thu, 18 Aug 2016 22:03:13 -0700
From:   Steve Muckle <steve.muckle@...aro.org>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Steve Muckle <steve.muckle@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux PM list <linux-pm@...r.kernel.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Juri Lelli <Juri.Lelli@....com>,
        Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [PATCH] sched: fix incorrect PELT values on SMT

On Fri, Aug 19, 2016 at 10:30:36AM +0800, Wanpeng Li wrote:
> 2016-08-19 9:55 GMT+08:00 Steve Muckle <steve.muckle@...aro.org>:
> > PELT scales its util_sum and util_avg values via
> > arch_scale_cpu_capacity(). If that function is passed the CPU's sched
> > domain then it will reduce the scaling capacity if SD_SHARE_CPUCAPACITY
> > is set. PELT does not pass in the sd however. The other caller of
> > arch_scale_cpu_capacity, update_cpu_capacity(), does. This means
> > util_sum and util_avg scale beyond the CPU capacity on SMT.
> >
> > On an Intel i7-3630QM for example rq->cpu_capacity_orig is 589 but
> > util_avg scales up to 1024.
> >
> > Fix this by passing in the sd in __update_load_avg() as well.
> 
> I believe we notice this at least several months ago.
> https://lkml.org/lkml/2016/5/25/228

Glad to see I'm not alone in thinking this is an issue.

It causes an issue with schedutil, effectively doubling the apparent
demand on SMT. I don't know the load balance code well enough offhand to
say whether it's an issue there.

cheers,
Steve