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.
next prev parent 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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=8bfd13a7-ed02-00dd-63a1-7144f2e55ef0@igalia.com \
--to=andrealmeid@igalia.com \
--cc=andrey.semashev@gmail.com \
--cc=bigeasy@linutronix.de \
--cc=dave@stgolabs.net \
--cc=dvhart@infradead.org \
--cc=fweimer@redhat.com \
--cc=libc-alpha@sourceware.org \
--cc=linux-api@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=peterz@infradead.org \
--cc=rostedt@goodmis.org \
--cc=tglx@linutronix.de \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.