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

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Message-ID: <20160819204450.GB27677@cloud>
Date:   Fri, 19 Aug 2016 13:44:50 -0700
From:   Josh Triplett <josh@...htriplett.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.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>,
        Dave Watson <davejwatson@...com>,
        Linux Kernel Mailing List <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>,
        Ben Maurer <bmaurer@...com>,
        Steven 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 Fri, Aug 19, 2016 at 01:23:57PM -0700, Linus Torvalds wrote:
> On Fri, Aug 19, 2016 at 1:07 PM, Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
> >
> > Benchmarking various approaches for reading the current CPU number:
> 
> So I'd like to see the benchmarks of something that actually *does* something.
> 
> IOW, what's the bigger-picture "this is what it actually is useful
> for, and how it speeds things up".
> 
> Nobody gets a cpu number just to get a cpu number - it's not a useful
> thing to benchmark. What does getcpu() so much that we care?

The combination of CPU number and restartable sequence allows userspace
to write "per-CPU" rather than "per-thread" algorithms, just as the
kernel can.  The kernel can do that with preempt_disable().  Userspace
can do it with "tell me my CPU and restart me if preempted".

But yes, this needs a benchmark of, for instance, urcu implemented on
top of this, or some concrete data structure.