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

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Date:   Tue, 4 Oct 2022 15:48:34 -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



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