linux-kernel - Re: [PATCH v2] sched/fair: Correctly insert cfs

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Date:   Thu, 3 Jun 2021 15:12:35 +0200
From:   Odin Ugedal <odin@...d.al>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Odin Ugedal <odin@...d.al>, 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>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] sched/fair: Correctly insert cfs_rq's to list on unthrottle

Hi,

> Out of curiosity, why did you decide to use
> cfs_rq->tg_load_avg_contrib instead of !cfs_rq_is_decayed(cfs_rq)
> which is used to delete the cfs_rq from the list when updating blocked
> load ?

Well, the main reason was that it is currently (without the other in
flight patches) not safe to just use "cfs_rq_is_decayed" directly,
since that could result in
a situation where tg_load_avg_contrib!=0 while
cfs_rq_is_decayed()==true. I guess we can use cfs_rq_is_decayed() if
you prefer that,
and all the other PELT patches are merged. (This was initially why I
thought a new field was a simpler and more elegant solution to make
sure we book-keep correctly,
but when the PELT stuff is fixed properly, that should be no real
issue as long it works as we expect).

I was also thinking about the cfs_rq->nr_running part; is there a
chance of a situation where a cfs_rq->nr_running==1 and it has no
load, resulting in it being decayed and
removed from the list in "__update_blocked_fair"? I have not looked
properly at it, but just wondering if that is actually possible..

Also, out of curiosity, are there some implications of a situation
where tg_load_avg_contrib=0 while *_load!=0, or would that not cause
fairness issues?

Thanks
Odin