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

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Date:   Mon, 3 Oct 2022 12:27:18 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>, juri.lelli@...hat.com,
        vincent.guittot@...aro.org,
        Youssef Esmat <youssefesmat@...gle.com>,
        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

There's a lot to unwind so I will reply in pieces after spending some time
thinking about it, but just for this part:

On 10/3/2022 12:14 PM, Qais Yousef wrote:
>> In this case, there is no lock involved yet you have a dependency. But I don't
>> mean to sound depressing, and just because there are cases like this does not
>> mean we should not solve the lock-based ones. When I looked at Android, I saw
>> that it uses futex directly from Android Runtime code instead of using pthread.
>> So perhaps this can be trivially converted to FUTEX_LOCK_PI and then what we do
>> in the kernel will JustWork(Tm) ?
> I guess it will depend on individual libc implementation, but I thought all of
> them use FUTEX under the hood for pthreads mutexes.
> 
> Maybe we can add a bootparam to force all futexes to be FUTEX_LOCK_PI?
> 

In the case of FUTEX_LOCK_PI, you have to store the TID of the 'lock owner' in
the futex word to signify that lock is held.

That wont work for the case above, Producer/Consumer signalling each other on a
bounded-buffer, right? That's not locking even though it is acquiring and
release of a limited resource.

thanks,

 - Joel