Re: Sum of weights idea for CFS PI

From: Joel Fernandes
Date: Tue Oct 04 2022 - 15:48:45 EST

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.


- Joel