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

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Message-Id: <F9BCD313-15BA-429C-B50C-1391DDC1239B@joelfernandes.org>
Date:   Wed, 19 Oct 2022 08:23:39 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Qais Yousef <qyousef@...alina.io>
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 Oct 19, 2022, at 7:43 AM, Qais Yousef <qyousef@...alina.io> wrote:
> 
> 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

Thanks for the pointer! Glad to see the consensus was that this is not acceptable.

I think we ought to write a patch to fix the accounting, for this series. I propose adding 2 new entries to proc/pid/stat which I think Juri was also sort of was alluding to:

1. Donated time.
2. Proxied time.

User space can then add or subtract this, to calculate things correctly. Or just display them in new columns. I think it will also actually show how much the proxying is happening for a use case.

Thoughts?

Thanks.


> Cheers
> 
> --
> Qais Yousef