linux-kernel - Re: [PATCH v3 4/5] sched/pelt: Add a new runnable average signal

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Date:   Fri, 21 Feb 2020 09:04:48 +0000
From:   Mel Gorman <mgorman@...e.de>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Phil Auld <pauld@...hat.com>, Parth Shah <parth@...ux.ibm.com>,
        Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH v3 4/5] sched/pelt: Add a new runnable average signal

On Thu, Feb 20, 2020 at 04:11:18PM +0000, Valentin Schneider wrote:
> On 20/02/2020 14:36, Vincent Guittot wrote:
> > I agree that setting by default to SCHED_CAPACITY_SCALE is too much
> > for little core.
> > The problem for little core can be fixed by using the cpu capacity instead
> > 
> 
> So that's indeed better for big.LITTLE & co. Any reason however for not
> aligning with the initialization of util_avg ?
> 
> With the default MC imbalance_pct (117), it takes 875 utilization to make
> a single CPU group (with 1024 capacity) overloaded (group_is_overloaded()).
> For a completely idle CPU, that means forking at least 3 tasks (512 + 256 +
> 128 util_avg)
> 
> With your change, it only takes 2 tasks. I know I'm being nitpicky here, but
> I feel like those should be aligned, unless we have a proper argument against
> it - in which case this should also appear in the changelog with so far only
> mentions issues with util_avg migration, not the fork time initialization.
> 

So, what is the way forward here? Should this patch be modified now,
a patch be placed on top or go with what we have for the moment that
works for symmetric CPUs and deal with the asym case later?

I do not have any asym systems at all so I've no means of checking
whether there is a problem or not.

-- 
Mel Gorman
SUSE Labs