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

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Message-ID: <4c52d81f-4b3b-d7e8-c124-b90b4584a6d3@arm.com>
Date:   Wed, 30 Oct 2019 18:25:09 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Phil Auld <pauld@...hat.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 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")

> 
> 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
> 
> 
>