Messages in this thread | | | Date | Tue, 4 Oct 2022 15:48:34 -0400 | Subject | Re: Sum of weights idea for CFS PI | From | Joel Fernandes <> |
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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
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