linux-kernel - Re: [RFC PATCH v8 1/9] Restartable sequences system call

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Message-ID: <20160827042226.akfaq4wku2gdpxsi@x>
Date:   Fri, 26 Aug 2016 21:22:26 -0700
From:   Josh Triplett <josh@...htriplett.org>
To:     Ben Maurer <bmaurer@...com>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Watson <davejwatson@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andrew Hunter <ahh@...gle.com>,
        Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
        rostedt <rostedt@...dmis.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RFC PATCH v8 1/9] Restartable sequences system call

On Thu, Aug 25, 2016 at 05:56:25PM +0000, Ben Maurer wrote:
> rseq opens up a whole world of algorithms to userspace – algorithms
> that are O(num CPUs) and where one can have an extremely fast fastpath
> at the cost of a slower slow path. Many of these algorithms are in use
> in the kernel today – per-cpu allocators, RCU, light-weight reader
> writer locks, etc. Even in cases where these APIs can be implemented
> today, a rseq implementation is often superior in terms of
> predictability and usability (eg per-thread counters consume more
> memory and are more expensive to read than per-cpu counters).
>
> Isn’t the large number of uses of rseq-like algorithms in the kernel a
> pretty substantial sign that there would be demand for similar
> algorithms by user-space systems programmers?

Yes and no.  It provides a substantial sign that such algorithms could
and should exist; however "someone should do this" doesn't demonstrate
that someone *will*.  I do think we need a concrete example of a
userspace user with benchmark numbers that demonstrate the value of this
approach.

Mathieu, do you have a version of URCU that can use rseq to work per-CPU
rather than per-thread?  URCU's data structures would work as a
benchmark.

Ben, Mathieu, Dave, do you have jemalloc benchmark numbers with and
without rseq?  (As well as memory usage numbers for the reduced memory
usage of per-CPU pools rather than per-thread pools?)