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

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Message-Id: <158BE4D6-039A-4D0B-B4DA-0FF7E35BCA1A@joelfernandes.org>
Date:   Thu, 20 Oct 2022 04:51:17 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Qais Yousef <qyousef@...alina.io>
Cc:     Juri Lelli <juri.lelli@...hat.com>,
        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>,
        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 3:30 PM, Qais Yousef <qyousef@...alina.io> wrote:
> 
> On 10/19/22 15:41, Juri Lelli wrote:
>>> On 19/10/22 08:23, Joel Fernandes wrote:
>>> 
>>> 
>>>> 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.
>> 
>> Sounds like a useful addition, at least from a debugging point of view.
> 
> They look useful addition to me too.

Thanks.

>>> 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.
>> 
>> Guess we'll however need to be backward compatible with old userspace?
>> Probably reporting the owner as running while proxied (as in the
>> comparison case vs. rtmutexes Valentin showed).
>> 
> 
> Or invent a new task_state? Doesn't have to be a real one, just report a new
> letter for tasks in PE state. We could use 'r' to indicate running BUT..

This is a good idea, especially for tracing. I still feel the time taken in the state
is also important to add so that top displays percentage properly.

 Best,

 -J

> Cheers
> 
> --
> Qais Yousef