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

Hi André!

On Thu, Mar 4, 2021 at 10:58 AM André Almeida [off-list ref] wrote:

Hi Peter,

Às 02:44 de 04/03/21, Peter Oskolkov escreveu:

quoted

On Wed, Mar 3, 2021 at 5:22 PM André Almeida [off-list ref] wrote:

quoted

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.