Messages in this thread | | | Date | Tue, 4 Oct 2022 10:43:33 -0400 | Subject | Re: Sum of weights idea for CFS PI | From | Joel Fernandes <> |
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On 10/4/2022 7:50 AM, Sebastian Andrzej Siewior wrote: > On 2022-09-30 13:34:49 [-0400], Joel Fernandes 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) ? > > The easy part is just to replace the lock/unlock functions with > FUTEX_LOCK_PI/UNLOCK_PI syscalls. The slightly advanced part is where > you use an atomic operation to replace 0 with threads's ID in the lock > path to avoid going into the kernel for locking if the lock is not > contended. If it is, then you need to use the syscall. > > … >>> Proxy execution seems to be the nice solution to all of these problems, but >>> it's a long way away. I'm interested to learn how this inheritance will be >>> implemented. And whether there are any userspace conversion issues. i.e: do >>> we need to convert all locks to rt-mutex locks? >> >> I am not an expert on FUTEX_LOCK_PI and this could be a good time for tglx to >> weigh in, but I think converting all userspace locks to use FUTEX_LOCK_PI sounds >> reasonable to me. > > Based on my understanding with proxy-execution, all in-kernel locks > should be covered. > Priority inheritance (PI) works only with FUTEX_LOCK_PI for userpace and > rtmutex for the in-kernel locks. Regular FUTEX_LOCK does only wait/wake > in userspace so there is no way for the kernel to "help". Ah and for PI > to work you need priorities that you can inherit. With two threads > running as SCHED_OTHER there will be just "normal" sleep+wake in the > kernel. If the blocking thread is SCHED_FIFO then it will inherit its > priority to the lock owner.
Hi Sebastian, I agree with your thoughts on this. Yes proxy execution idea should cover this. Basically, any primitive that allows userspace to let the kernel know is a dependency can use this AFAICS, FUTEX_LOCK_PI being a prime example. Perhaps Android's binder being another where A sends a message to C and blocks till C responds. Meanwhile medium prio B blocks C.
thanks,
- Joel
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