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Date: Fri, 19 Aug 2016 13:44:50 -0700
From: Josh Triplett <josh@joshtriplett.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Peter Zijlstra <peterz@infradead.org>,
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Subject: Re: [RFC PATCH v8 1/9] Restartable sequences system call
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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@efficios.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.