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

[Peter Oskolkov <posk@posk.io>]

On Fri, Mar 5, 2021 at 12:03 PM Peter Oskolkov <posk@posk.io> wrote:
>
> Hi André!
>
> On Thu, Mar 4, 2021 at 10:58 AM André Almeida <andrealmeid@collabora.com> 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@collabora.com> 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;
> }

Or an even more complete example:

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

   /* wait for the consumer */
   prev = next;
   do {
     next = atomic_get(&futex);
     futex_wait(&futex, prev);
  } while (next != NULL);

}

T2 (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;
    atomic_set(&futex, NULL);
    futex_wake(&futex, 1); /* signal we can consumer more */
}

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