Re: Sum of weights idea for CFS PI

From: Joel Fernandes <joel@joelfernandes.org>
To: Qais Yousef <qais.yousef@arm.com>
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 15:48:34 -0400	[thread overview]
Message-ID: <0773b853-afdf-4368-3494-a5bd1cf01893@joelfernandes.org> (raw)
In-Reply-To: <20221004163039.vv3byszpg5dqjhy5@wubuntu>

On 10/4/2022 12:30 PM, Qais Yousef wrote:
> 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.

I don't disagree. Proxy execution is an implementation detail, without more
information from userspace, any implementation cannot help. I was just
responding to your point about converting all futexes which you cannot do
without knowing what the futex is used for.

But I am thinking of messing around with rt_mutex_setprio() and some userspace
tests to see if I can make the sum of weights thing work for the *userspace
locking* usecases (FUTEX_LOCK_PI). Then run some tests and collect some traces.
Perhaps you can do that on the Android side as well.

> 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.
Most certainly, agreed.

Thanks!

 - Joel