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From:   Peter Oskolkov <posk@posk.io>
Date:   Thu, 23 Jul 2020 20:22:14 -0700
Message-ID: <CAFTs51XVbZ4y5NrHrcfBBb5shrQRcX4y8SAjvm76T_=EbxDiYA@mail.gmail.com>
Subject: Re: [PATCH for 5.9 1/3] futex: introduce FUTEX_SWAP operation
To:     Waiman Long <longman@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Ingo Molnar <mingo@kernel.org>,
        Darren Hart <dvhart@infradead.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Peter Oskolkov <posk@google.com>,
        Andrei Vagin <avagin@google.com>, Paul Turner <pjt@google.com>,
        Ben Segall <bsegall@google.com>, Aaron Lu <aaron.lwe@gmail.com>
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On Thu, Jul 23, 2020 at 8:00 PM Waiman Long <longman@redhat.com> wrote:
>
> On 7/23/20 8:25 PM, Peter Oskolkov wrote:
> > On Thu, Jul 23, 2020 at 4:28 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > Thanks a lot for your comments, Peter! My answers below.
> >
> >> On Wed, Jul 22, 2020 at 04:45:36PM -0700, Peter Oskolkov wrote:
> >>> This patchset is the first step to open-source this work. As explained
> >>> in the linked pdf and video, SwitchTo API has three core operations: wait,
> >>> resume, and swap (=switch). So this patchset adds a FUTEX_SWAP operation
> >>> that, in addition to FUTEX_WAIT and FUTEX_WAKE, will provide a foundation
> >>> on top of which user-space threading libraries can be built.
> >> The PDF and video can go pound sand; you get to fully explain things
> >> here.
> > Will do. Should I expand the cover letter or the commit message? (I'll probably
> > split the first patch into two in the latter case).
>
> You should put it mostly in the commit message which will be part of the
> git log history. The cover letter, on the other hand, is not part of the
> git log.

Ack. (Networking/David Miller usually includes the cover letter in the
git log, so this
is context dependent, I guess).

>
>
> >
> >> What worries me is how FUTEX_SWAP would interact with the future
> >> FUTEX_LOCK / FUTEX_UNLOCK. When we implement pthread_mutex with those,
> >> there's very few WAIT/WAKE left.
> > [+cc Waiman Long]
> >
> > I've looked through the latest FUTEX_LOCK patchset I could find (
> > https://lore.kernel.org/patchwork/cover/772643/ and related), and it seems
> > that FUTEX_SWAP and FUTEX_LOCK/FUTEX_UNLOCK patchsets
> > address the same issue (slow wakeups) but for different use cases:
> >
> > FUTEX_LOCK/FUTEX_UNLOCK uses spinning and lock stealing to
> > improve futex wake/wait performance in high contention situations;
> > FUTEX_SWAP is designed to be used for fast context switching with
> > _no_ contention by design: the waker that is going to sleep, and the wakee
> > are using different futexes; the userspace will have a futex per thread/task,
> > and when needed the thread/task will either simply sleep on its futex,
> > or context switch (=FUTEX_SWAP) into a different thread/task.
>
> I agree that it is a different use case. I just hope that you keep the
> possible future extension to support FUTEX_LOCK/UNLOCK in mind so that
> they won't conflict with each other.

Ack. Will do. :)

Thanks,
Peter

>
> Cheers,
> Longman
>