linux-kernel - Re: [RFC] futex2: add NUMA awareness

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Message-ID: <8bfd13a7-ed02-00dd-63a1-7144f2e55ef0@igalia.com>
Date:   Thu, 14 Jul 2022 12:00:59 -0300
From:   André Almeida <andrealmeid@...lia.com>
To:     Andrey Semashev <andrey.semashev@...il.com>
Cc:     linux-api@...r.kernel.org, fweimer@...hat.com,
        linux-kernel@...r.kernel.org, Darren Hart <dvhart@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        libc-alpha@...rceware.org, Davidlohr Bueso <dave@...olabs.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [RFC] futex2: add NUMA awareness

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.

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

Let me know if this clarifies your questions.