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

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Message-ID: <Y0/+ieCymugrjheC@localhost.localdomain>
Date:   Wed, 19 Oct 2022 15:41:29 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Qais Yousef <qyousef@...alina.io>,
        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 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.

> 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).