Re: [RFC PATCH v2 00/13] Add futex2 syscall

[Peter Oskolkov]

Hi André!

On Thu, Mar 4, 2021 at 10:58 AM André Almeida <andrealmeid@xxxxxxxxxxxxx> wrote:
>
> Hi Peter,
>
> Às 02:44 de 04/03/21, Peter Oskolkov escreveu:
> > On Wed, Mar 3, 2021 at 5:22 PM André Almeida <andrealmeid@xxxxxxxxxxxxx> wrote:
> >>
> >> Hi,
> >>
> >> This patch series introduces the futex2 syscalls.
> >>
> >> * FAQ
> >>
> >>   ** "And what's about FUTEX_64?"
> >>
> >>   By supporting 64 bit futexes, the kernel structure for futex would
> >>   need to have a 64 bit field for the value, and that could defeat one of
> >>   the purposes of having different sized futexes in the first place:
> >>   supporting smaller ones to decrease memory usage. This might be
> >>   something that could be disabled for 32bit archs (and even for
> >>   CONFIG_BASE_SMALL).
> >>
> >>   Which use case would benefit for FUTEX_64? Does it worth the trade-offs?
> >
> > The ability to store a pointer value on 64bit platforms is an
> > important use case.
> > Imagine a simple producer/consumer scenario, with the producer updating
> > some shared memory data and waking the consumer. Storing the pointer
> > in the futex makes it so that only one shared memory location needs to be
> > accessed "atomically", etc. With two atomics synchronization becomes
> > more involved (= slower).
> >
>
> So the idea is to, instead of doing this:
>
> T1:
> atomic_set(&shm_addr, buffer_addr);
> atomic_set(&futex, 0);
> futex_wake(&futex, 1);
>
> T2:
> consume(shm_addr);
>
> To do that:
>
> T1:
> atomic_set(&futex, buffer_addr);
> futex_wake(&futex, 1);
>
> T2:
> consume(futex);
>
> Right?

More like this:

T1 (producer):
while (true) {
    ptr = get_new_data();
    atomic_set(&futex, ptr);
    futex_wake(&futex, 1);
}

T1 (consumer):
some_data *prev = NULL;
while (true) {
  futex_wait(&futex, prev);
  some_data *next = atomic_get(&futex);
  if (next == prev) continue;  /* spurious wakeup */

  consume_data(next);
  prev = next;
}



>
> I'll try to write a small test to see how the perf numbers looks like.