linux-kernel - Re: [PATCH v4 00/10] sched/fair: rework the CFS load balance

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Message-ID: <20191030172920.GH1686@pauld.bos.csb>
Date:   Wed, 30 Oct 2019 13:29:20 -0400
From:   Phil Auld <pauld@...hat.com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Quentin Perret <quentin.perret@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        Hillf Danton <hdanton@...a.com>,
        Parth Shah <parth@...ux.ibm.com>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v4 00/10] sched/fair: rework the CFS load balance

On Wed, Oct 30, 2019 at 06:25:09PM +0100 Valentin Schneider wrote:
> On 30/10/2019 18:19, Phil Auld wrote:
> >> Well from the code nobody but us (asymmetric capacity systems) set
> >> SD_BALANCE_WAKE. I was however curious if there were some folks who set it
> >> with out of tree code for some reason.
> >>
> >> As Dietmar said, not having SD_BALANCE_WAKE means you'll never go through
> >> the slow path on wakeups, because there is no domain with SD_BALANCE_WAKE for
> >> the domain loop to find. Depending on your topology you most likely will
> >> go through it on fork or exec though.
> >>
> >> IOW wake_wide() is not really widening the wakeup scan on wakeups using
> >> mainline topology code (disregarding asymmetric capacity systems), which
> >> sounds a bit... off.
> > 
> > Thanks. It's not currently set. I'll set it and re-run to see if it makes
> > a difference. 
> > 
> 
> Note that it might do more harm than good, it's not set in the default
> topology because it's too aggressive, see 
> 
>   182a85f8a119 ("sched: Disable wakeup balancing")
> 

Heh, yeah... even as it's running I can see that this killing it :)


> > 
> > However, I'm not sure why it would be making a difference for only the cgroup
> > case. If this is causing issues I'd expect it to effect both runs. 
> > 
> > In general I think these threads want to wake up the last cpu they were on.
> > And given there are fewer cpu bound tasks that CPUs that wake cpu should,
> > more often than not, be idle. 
> > 
> > 
> > Cheers,
> > Phil
> > 
> > 
> > 

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