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From:   Odin Ugedal <odin@uged.al>
Date:   Thu, 27 May 2021 13:04:18 +0200
Message-ID: <CAFpoUr0PTYs+CSiWt3WOXnxq=wN3uEyC=h+_3kDc9wLoqaRC_Q@mail.gmail.com>
Subject: Re: [PATCH 1/3] sched/fair: Add tg_load_contrib cfs_rq decay checking
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     Odin Ugedal <odin@uged.al>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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> 1st : ensure that cfs_rq->load_sum is not null if cfs_rq-> load_isn't too
> 2nd : call update_tg_load_avg() during child update so we will be sure
> to update tg_load_avg_contrib before removing the cfs from the list

Ahh, yeah, with "1st" that would work. Yeah, that was my initial
implementation of the change, but I thought that it was better to keep
the logic away from the "hot path". We can verify this in
update_tg_cfs_load(), and then force update_tg_load_avg() inside
__update_blocked_fair() when avg.load_avg is 0. (Given that this is
the only place where we can end up in this situation. I can update
this patch to do that instead.

Another solution is to update avg.load_avg
inside__update_blocked_fair() when load_sum is 0, and then propagate
that with update_tg_load_avg(). This removes the logic from the hot
path all together.

Not sure what the preferred way is. I have not found any other places
where this situation _should_ occur, but who knows..

Odin