Re: Sum of weights idea for CFS PI

From: Qais Yousef <qais.yousef@arm.com>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	juri.lelli@redhat.com, vincent.guittot@linaro.org,
	Youssef Esmat <youssefesmat@google.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	bristot@redhat.com, clark.williams@gmail.com,
	bigeasy@linutronix.de, "Paul E. McKenney" <paulmck@kernel.org>
Subject: Re: Sum of weights idea for CFS PI
Date: Tue, 4 Oct 2022 17:30:39 +0100	[thread overview]
Message-ID: <20221004163039.vv3byszpg5dqjhy5@wubuntu> (raw)
In-Reply-To: <160a2ded-b8e0-acf0-a8b6-df1b0f2c0fa8@joelfernandes.org>

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