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In-Reply-To: <1641894961-9241-2-git-send-email-CruzZhao@linux.alibaba.com>
From:   Josh Don <joshdon@google.com>
Date:   Tue, 11 Jan 2022 15:52:02 -0800
Message-ID: <CABk29NuX1XYUXj8uZrSjm83n=-uk1LUbRQSMpo2s6er2pTRmDQ@mail.gmail.com>
Subject: Re: [PATCH v2 1/3] sched/core: Accounting forceidle time for all
 tasks except idle task
To:     Cruz Zhao <CruzZhao@linux.alibaba.com>
Cc:     Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Benjamin Segall <bsegall@google.com>,
        Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        cgroups@vger.kernel.org,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Tue, Jan 11, 2022 at 1:56 AM Cruz Zhao <CruzZhao@linux.alibaba.com> wrote:
>
> There are two types of forced idle time: forced idle time from cookie'd
> task and forced idle time form uncookie'd task. 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 both.
>
> Assuming cpu x and cpu y are a pair of SMT siblings, consider the
> following scenarios:
>   1.There's a cookie'd task running on cpu x, and there're 4 uncookie'd
>     tasks 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 running on cpu x, and there're 4 cookie'd
>     tasks 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 total capacity loss is 1 cpu, but in
> scenario1, the cookie'd forced idle time tells us 20% cpu capacity loss, in
> scenario2, the cookie'd forced idle time tells us 80% cpu capacity loss,
> which are not accurate. It'll be more accurate to measure with cookie'd
> forced idle time and uncookie'd forced idle time.
>
> Signed-off-by: Cruz Zhao <CruzZhao@linux.alibaba.com>
> ---

Thanks,

Reviewed-by: Josh Don <joshdon@google.com>