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From:   Vineeth Remanan Pillai <vpillai@digitalocean.com>
Date:   Mon, 29 Jun 2020 15:41:04 -0400
Message-ID: <CANaguZB1_wBMXMzb=itEfhhYTh6F0GbMT_1OWmw_XS-ufi7Umg@mail.gmail.com>
Subject: Re: [RFC PATCH 00/13] Core scheduling v5
To:     "Li, Aubrey" <aubrey.li@linux.intel.com>
Cc:     Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
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        Ingo Molnar <mingo@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Greg Kerr <kerrnel@google.com>, Phil Auld <pauld@redhat.com>,
        Aaron Lu <aaron.lwe@gmail.com>,
        Aubrey Li <aubrey.intel@gmail.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
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        Paul Turner <pjt@google.com>
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Hi Aubrey,

On Mon, Jun 29, 2020 at 8:34 AM Li, Aubrey <aubrey.li@linux.intel.com> wrote:
>
> >  - Load balancing/migration changes ignores group weights:
> >    - https://lwn.net/ml/linux-kernel/20200225034438.GA617271@ziqianlu-desktop.localdomain
>
> According to Aaron's response below:
> https://lwn.net/ml/linux-kernel/20200305085231.GA12108@ziqianlu-desktop.localdomain/
>
> The following logic seems to be helpful for Aaron's case.
>
> +       /*
> +        * Ignore cookie match if there is a big imbalance between the src rq
> +        * and dst rq.
> +        */
> +       if ((src_rq->cfs.h_nr_running - rq->cfs.h_nr_running) > 1)
> +               return true;
>
> I didn't see any other comments on the patch at here:
> https://lwn.net/ml/linux-kernel/67e46f79-51c2-5b69-71c6-133ec10b68c4@linux.intel.com/
>
> Do we have another way to address this issue?
>
We do not have a clear fix for this yet, and did not get much time to
work on this.

I feel that the above change would not be fixing the real issue.
The issue is about not considering the weight of the group when we
try to load balance, but the above change is checking only the
nr_running which might not work always. I feel that we should fix
the real issue in v6 and probably hold on to adding the workaround
fix in the interim.  I have added a TODO specifically for this bug
in v6.

What do you think?

Thanks,
Vineeth