linux-kernel - Re: [PATCH 2/2] sched/core: Uncookied force idle accounting per cpu

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Date:   Wed, 5 Jan 2022 12:59:27 -0800
From:   Josh Don <joshdon@...gle.com>
To:     cruzzhao <cruzzhao@...ux.alibaba.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 2/2] sched/core: Uncookied force idle accounting per cpu

On Wed, Jan 5, 2022 at 3:33 AM cruzzhao <cruzzhao@...ux.alibaba.com> wrote:
>
> When we care about capacity loss, we care about all but not some of it.
> The forced idle time from uncookie'd task is actually caused by the
> cookie'd task in runqueue indirectly, and it's more accurate to measure
> the capacity loss with the sum of cookie'd forced idle time and
> uncookie'd forced idle time, as far as I'm concerned.
>
> Assuming cpu x and cpu y are a pair of smt siblings, consider the
> following scenarios:
> 1. There's a cookie'd task A running on cpu x, and there're 4 uncookie'd
> tasks B~E running on cpu y. For cpu x, there will be 80% forced idle
> time(from uncookie'd task); for cpu y, there will be 20% forced idle
> time(from cookie'd task).
> 2. There's a uncookie'd task A running on cpu x, and there're 4 cookie'd
> tasks B~E running on cpu y. For cpu x, there will be 80% forced idle
> time(from cookie'd task); for cpu y, there will be 20% forced idle
> time(from uncookie'd task).
> The scenario1 can recurrent by stress-ng(scenario2 can recurrent similary):
> (cookie'd)taskset -c x stress-ng -c 1 -l 100
> (uncookie'd)taskset -c y stress-ng -c 4 -l 100
>
> In the above two scenarios, the capacity loss is 1 cpu, but in
> scenario1, the cookie'd forced idle time tells us 20%cpu loss, in
> scenario2, the cookie'd forced idle time tells us 80% forced idle time,
> which are not accurate. It'll be more accurate with the sum of cookie'd
> forced idle time and uncookie'd forced idle time.

Why do you need this separated out into two fields then? Could we just
combine the uncookie'd and cookie'd forced idle into a single sum?

IMO it is fine to account the forced idle from uncookie'd tasks, but
we should then also change the task accounting to do the same, for
consistency.