linux-kernel - Re: [PATCH v5 2/3] sched/fair: Add cfs bandwidth burst statistics

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Date:   Thu, 20 May 2021 16:11:52 +0200
From:   Odin Ugedal <odin@...d.al>
To:     Huaixin Chang <changhuaixin@...ux.alibaba.com>
Cc:     Benjamin Segall <bsegall@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        dtcccc@...ux.alibaba.com, Juri Lelli <juri.lelli@...hat.com>,
        khlebnikov@...dex-team.ru,
        open list <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...e.de>, Ingo Molnar <mingo@...hat.com>,
        Odin Ugedal <odin@...d.al>, pauld@...head.com,
        Peter Zijlstra <peterz@...radead.org>,
        Paul Turner <pjt@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        shanpeic@...ux.alibaba.com, Tejun Heo <tj@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        xiyou.wangcong@...il.com
Subject: Re: [PATCH v5 2/3] sched/fair: Add cfs bandwidth burst statistics

I am a bit sceptical about both the nr_burst and burst_time as they are now.

As an example; a control group using "99.9%" of the quota each period
and that is never throttled. Such group would with this patch with a burst of X
still get nr_throttled = 0 (as before), but it would get a nr_burst
and burst_time that
will keep increasing.

I think there is a big difference between runtime moved/taken from
cfs_b->runtime to cfs_rq->runtime_remaining and the actual runtime used
in the period. Currently, cfs bw can only supply info the first one, and
not the latter.

I think that if people see nr_burst increasing, that they think they _have_
to use cfs burst in order to avoid being throttled, even though that might
not be the case. It is probably fine as is, as long as it is explicitly stated
what the values mean and imply, and what they do not. I cannot see another
way to calculate it as it is now, but maybe someone else has some thoughts.

Thanks
Odin