linux-kernel - Re: Sum of weights idea for CFS PI

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Message-ID: <CAEXW_YQQ=CfQEOkXK-VFJtZTdY=_TsWz=Ndgg_BgJumFHJktkQ@mail.gmail.com>
Date:   Fri, 30 Sep 2022 14:45:10 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Youssef Esmat <youssefesmat@...gle.com>
Cc:     Qais Yousef <qais.yousef@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>, juri.lelli@...hat.com,
        vincent.guittot@...aro.org,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Thomas Gleixner <tglx@...utronix.de>, bristot@...hat.com,
        clark.williams@...il.com, bigeasy@...utronix.de,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: Sum of weights idea for CFS PI

On Fri, Sep 30, 2022 at 2:10 PM Youssef Esmat <youssefesmat@...gle.com> wrote:
[..]
> > > Hi Everyone!
> >
> > Hi Youssef,
> >
> > (Youssef is new to LKML though in no way new to OS or software development. I
> > gave him the usual 'dont-top-post' chat already - fyi).
> >
> > > I am not sure we should care about A's sleeping pattern. The case we
> > > care about is when A is running or wants to run but can't because it
> > > is blocked on C. In that case C should get the weight of A as if A was
> > > running.
> >
> > Just to clarify - Youssef did mean sum of weights of different things in the
> > chain, and not just weights (he confirmed on chat that that's what he meant).
> >
>
> Yeah thanks for clarifying, I meant that C should get the sum of
> weights as if A was running (3/5 in your example) since in this
> segment of time A would have been running if it was not blocked on the
> lock. I think it's safe to ignore the average and just use the sum of

For the onlooker, we are talking about the classical case of priority
inversion involving 3 tasks A, B and C which can be expanded to a
chain of tasks. Highest prio A blocks on a lock that lowest prio C
holds, while an unrelated medium prio B blocks C (or reduces progress
of it as in the case of CFS).

On the note of "A would have been running if it was not blocked on the
lock". I think that would be an assumption - we don't know if A would
be running. We only know the past, not the future. A could very well
make an I/O request for example. Hence there could be a need to use
A's past utilization, right?

thanks,

 - Joel