linux-kernel - Re: [RFC PATCH 00/11] Reviving the Proxy Execution Series

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Date:   Wed, 19 Oct 2022 12:43:57 +0100
From:   Qais Yousef <qyousef@...alina.io>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Connor O'Brien <connoro@...gle.com>, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com, John Stultz <jstultz@...gle.com>,
        Qais Yousef <qais.yousef@....com>,
        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>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [RFC PATCH 00/11] Reviving the Proxy Execution Series

On 10/17/22 02:23, Joel Fernandes wrote:

> I ran a test to check CFS time sharing. The accounting on top is confusing,
> but ftrace confirms the proxying happening.
> 
> Task A - pid 122
> Task B - pid 123
> Task C - pid 121
> Task D - pid 124
> 
> Here D and B just spin all the time. C is lock owner (in-kernel mutex) and
> spins all the time, while A blocks on the same in-kernel mutex and remains
> blocked.
> 
> Then I did "top -H" while the test was running which gives below output.
> The first column is PID, and the third-last column is CPU percentage.
> 
> Without PE:
>   121 root      20   0   99496   4   0 R  33.6   0.0   0:02.76 t  (task C)
>   123 root      20   0   99496   4   0 R  33.2   0.0   0:02.75 t  (task B)
>   124 root      20   0   99496   4   0 R  33.2   0.0   0:02.75 t  (task D)
> 
> With PE:
>   PID
>   122 root      20   0   99496   4   0 D  25.3   0.0   0:22.21 t  (task A)
>   121 root      20   0   99496   4   0 R  25.0   0.0   0:22.20 t  (task C)
>   123 root      20   0   99496   4   0 R  25.0   0.0   0:22.20 t  (task B)
>   124 root      20   0   99496   4   0 R  25.0   0.0   0:22.20 t  (task D)
> 
> With PE, I was expecting 2 threads with 25% and 1 thread with 50%. Instead I
> get 4 threads with 25% in the top. Ftrace confirms that the D-state task is
> in fact not running and proxying to the owner task so everything seems
> working correctly, but the accounting seems confusing, as in, it is confusing
> to see the D-state task task taking 25% CPU when it is obviously "sleeping".
> 
> Yeah, yeah, I know D is proxying for C (while being in the uninterruptible
> sleep state), so may be it is OK then, but I did want to bring this up :-)

I seem to remember Valentin raised similar issue about how userspace view can
get confusing/misleading:

	https://www.youtube.com/watch?v=UQNOT20aCEg&t=3h21m41s


Cheers

--
Qais Yousef