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

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Message-Id: <0BFD3887-60A2-4C74-9D37-49B7B6E64299@joelfernandes.org>
Date:   Sat, 8 Oct 2022 11:04:52 -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 Oct 6, 2022, at 3:40 PM, Youssef Esmat <youssefesmat@...gle.com> wrote:
> 
[..]
>> 
>>> Anyway - just trying to explain how I see it and why C is unlikely to be taking
>>> too much time. I could be wrong. As Youssef said, I think there's no
>>> fundamental problem here.
>> 
>> I know on Android where they use smaller HZ, the large tick causes
>> lots of problems for large nice deltas. Example if a highly niced task
>> was to be preempted for 1ms, and preempts instead at 3ms, then the
>> less-niced task will not be so nice (even less nice than it promised
>> to be) any more because of the 2ms boost that the higher niced task
>> got. This can lead the the sched_latency thrown out of the window. Not
>> adjusting the weights properly can potentially make that problem much
>> worse IMO.
> 
> Once C releases the lock it should get adjusted and A will get
> adjusted also regardless of tick. At the point we adjust the weights
> we have a chance to check for preemption and cause a reschedule.

Yes but the lock can be held for potentially long time (and even user space lock). I’m more comfortable with Peter’s PE patch which seems a more generic solution, than sum of weights if we can get it working. I’m studying Connor’s patch set now…

> If C doesn't release the lock quickly (hopefully rare), it should
> continue to run at the adjusted weight since it is still blocking A.

We can’t depend on rare. Even one bad instance is a fail. So if lock holder and releaser go crazy, we can’t destabilize the system. After all, this is CFS and fairness should not be broken, even if rarely.

Thanks.


> 
>> 
>> Thanks.