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Date:   Wed, 17 May 2023 09:03:10 -0700
From:   Davidlohr Bueso <dave@stgolabs.net>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        "H . Peter Anvin" <hpa@zytor.com>, Paul Turner <pjt@google.com>,
        linux-api@vger.kernel.org, Christian Brauner <brauner@kernel.org>,
        Florian Weimer <fw@deneb.enyo.de>, David.Laight@ACULAB.COM,
        carlos@redhat.com, Peter Oskolkov <posk@posk.io>,
        Alexander Mikhalitsyn <alexander@mihalicyn.com>,
        Chris Kennelly <ckennelly@google.com>,
        Ingo Molnar <mingo@redhat.com>,
        Darren Hart <dvhart@infradead.org>,
        =?utf-8?B?QW5kcsOvwr/CvQ==?= Almeida <andrealmeid@igalia.com>,
        libc-alpha@sourceware.org, Steven Rostedt <rostedt@goodmis.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Florian Weimer <fweimer@redhat.com>
Subject: Re: [RFC PATCH 1/4] rseq: Add sched_state field to struct rseq
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 <20230517152654.7193-2-mathieu.desnoyers@efficios.com>
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On Wed, 17 May 2023, Mathieu Desnoyers wrote:

>Expose the "on-cpu" state for each thread through struct rseq to allow
>adaptative mutexes to decide more accurately between busy-waiting and
>calling sys_futex() to release the CPU, based on the on-cpu state of the
>mutex owner.

Oh yeah moving the spin stuff out of the kernel is much nicer.

>It is only provided as an optimization hint, because there is no
>guarantee that the page containing this field is in the page cache, and
>therefore the scheduler may very well fail to clear the on-cpu state on
>preemption. This is expected to be rare though, and is resolved as soon
>as the task returns to user-space.
>
>The goal is to improve use-cases where the duration of the critical
>sections for a given lock follows a multi-modal distribution, preventing
>statistical guesses from doing a good job at choosing between busy-wait
>and futex wait behavior.
>
>Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
>Cc: Jonathan Corbet <corbet@lwn.net>
>Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
>Cc: Carlos O'Donell <carlos@redhat.com>
>Cc: Florian Weimer <fweimer@redhat.com>
>Cc: libc-alpha@sourceware.org
>---
> include/linux/sched.h     | 12 ++++++++++++
> include/uapi/linux/rseq.h | 17 +++++++++++++++++
> kernel/rseq.c             | 14 ++++++++++++++
> 3 files changed, 43 insertions(+)

Ie: previous efforts

  kernel/futex.c             |  675 ++++++++++++++++++++++++++++++++++++++------
  kernel/futex.c             |  572 ++++++++++++++++++++++++++++++++++++-------------

Thanks,
Davidlohr