Re: [RFC] futex2: add NUMA awareness

[Andrey Semashev <andrey.semashev@gmail.com>]

On 7/14/22 18:00, André Almeida wrote:
> Hi Andrey,
> 
> Thanks for the feedback.
> 
> Às 08:01 de 14/07/22, Andrey Semashev escreveu:
>> On 7/14/22 06:18, André Almeida wrote:
> [...]
>>>
>>> Feedback? Who else should I CC?
>>
>> Just a few questions:
>>
>> Do I understand correctly that notifiers won't be able to wake up
>> waiters unless they know on which node they are waiting?
>>
> 
> If userspace is using NUMA_FLAG, yes. Otherwise all futexes would be
> located in the default node, and userspace doesn't need to know which
> one is the default.
> 
>> Is it possible to wait on a futex on different nodes?
> 
> Yes, given that you specify `.hint = id` with the proper node id.

So any given futex_wake(FUTEX_NUMA) operates only within its node, right?

>> Is it possible to wake waiters on a futex on all nodes? When a single
>> (or N, where N is not "all") waiter is woken, which node is selected? Is
>> there a rotation of nodes, so that nodes are not skewed in terms of
>> notified waiters?
> 
> Regardless of which node the waiter process is running, what matter is
> in which node the futex hash table is. So for instance if we have:
> 
> 	struct futex32_numa f = {.value = 0, hint = 2};
> 
> And now we add some waiters for this futex:
> 
> Thread 1, running on node 3:
> 
> 	futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL);
> 
> Thread 2, running on node 0:
> 
> 	futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL);
> 
> Thread 3, running on node 2:
> 
> 	futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL);
> 
> And then, Thread 4, running on node 3:
> 
> 	futex_wake(&f, 2, FUTEX_NUMA | FUTEX_32);
> 
> Now, two waiter would wake up (e.g. T1 and T3, node 3 and 2) and they
> are from different nodes. futex_wake() doesn't provide guarantees of
> which waiter will be selected, so I can't say which node would be
> selected.

In this example, T1, T2 and T3 are all blocking on node 2 (since all of
them presumably specify hint == 2), right? In this sense, it doesn't
matter which node they are running on, what matters is what node they
block on.

What I'm asking is can I wake all threads blocked on all nodes on the
same futex? That is, is the following possible?

  // I'm using hint == -1 to indicate the current node
  // of the calling thread for waiters and all nodes for notifiers
  struct futex32_numa f = {.value = 0, .hint = -1};

  Thread 1, running on node 3, blocks on node 3:

  futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL);

  Thread 2, running on node 0, blocks on node 0:

  futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL);

  Thread 3, running on node 2, blocks on node 2:

  futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL);

  And then, Thread 4, running on whatever node:

  futex_wake(&f, -1, FUTEX_NUMA | FUTEX_32);

Here, futex_wake would wake T1, T2 and T3. Or:

  futex_wake(&f, 1, FUTEX_NUMA | FUTEX_32);

Here, futex_wake would wake any one of T1, T2 or T3.

> There's no policy for fairness/starvation for futex_wake(). Do
> you think this would be important for the NUMA case?

I'm not sure yet. If there isn't a cross-node behavior like in my
example above then, I suppose, it falls to the userspace to ensure fair
rotation of the wakeups on different nodes. If there is functionality
like this, I imagine, some sort of fairness would be desired.