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

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Message-ID: <20221004163039.vv3byszpg5dqjhy5@wubuntu>
Date:   Tue, 4 Oct 2022 17:30:39 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Joel Fernandes <joel@...lfernandes.org>
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

On 10/03/22 12:27, Joel Fernandes wrote:
> 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.

Right. So userspace has to opt-in.

> 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.

Yes but as I tried to point out I don't think proxy-execution handles this case
where you don't hold a lock explicitly. But I could be wrong. IIUC Sebastian's
understanding is similar to mine. Only 'locks' (FUTEX_LOCK_PI which ends up
using rt-mutex) do PI inheritance.

So this signaling scenario is a new class of problems that wasn't handled
before; to my understanding.


Thanks

--
Qais Yousef