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From: "André Almeida" <andrealmeid@igalia.com>
To: Andrey Semashev <andrey.semashev@gmail.com>
Cc: linux-api@vger.kernel.org, fweimer@redhat.com,
	linux-kernel@vger.kernel.org, Darren Hart <dvhart@infradead.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	libc-alpha@sourceware.org, Davidlohr Bueso <dave@stgolabs.net>,
	Steven Rostedt <rostedt@goodmis.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: Re: [RFC] futex2: add NUMA awareness
Date: Thu, 14 Jul 2022 12:00:59 -0300	[thread overview]
Message-ID: <8bfd13a7-ed02-00dd-63a1-7144f2e55ef0@igalia.com> (raw)
In-Reply-To: <74ba5239-27b0-299e-717c-595680cd52f9@gmail.com>

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.

  reply	other threads:[~2022-07-14 15:01 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-07-14  3:18 [RFC] futex2: add NUMA awareness André Almeida
2022-07-14 11:01 ` Andrey Semashev
2022-07-14 15:00   ` André Almeida [this message]
2022-07-22 16:42     ` Andrey Semashev
2022-07-27 17:19       ` André Almeida

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