Re: [RFC][PATCH] spin loop arch primitives for busy waiting

From	Linus Torvalds <torvalds@linux-foundation.org>
Newsgroups	linux.kernel
Subject	Re: [RFC][PATCH] spin loop arch primitives for busy waiting
Date	2017-04-06 17:20 +0200
Message-ID	<tthmx-2d4-1@gated-at.bofh.it> (permalink)
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On Thu, Apr 6, 2017 at 7:13 AM, Will Deacon <will.deacon@arm.com> wrote:
>
> We've wrapped this up in the arm64 code as __cmpwait, and we use that
> to build smp_cond_load_acquire. It would be nice to use the same machinery
> for the conditional spinning here, unless you anticipate that we're only
> going to be spinning for a handful of iterations anyway?

I suspect most of these loops aren't set up for the WFE kind of
spinning, because they look for more than one variable.

.. and the ones that _are_ set up for this probably should just be
rewritten to use smp_cond_load_acquire() anyway, because the "wait for
value" special case is fairly special.

In theory x86 could use monitor/mwait for it too, in practice I think
it tends to still be too high latency (because it was originally just
designed for the idle loop). mwait got extended to actually be useful,
but I'm not sure what the latency is for the modern one.

               Linus

Thread

[RFC][PATCH] spin loop arch primitives for busy waiting Nicholas Piggin <npiggin@gmail.com> - 2017-04-03 10:20 +0200
  Re: [RFC][PATCH] spin loop arch primitives for busy waiting Linus Torvalds <torvalds@linux-foundation.org> - 2017-04-03 17:40 +0200
    Re: [RFC][PATCH] spin loop arch primitives for busy waiting Nicholas Piggin <npiggin@gmail.com> - 2017-04-04 02:00 +0200
      Re: [RFC][PATCH] spin loop arch primitives for busy waiting Linus Torvalds <torvalds@linux-foundation.org> - 2017-04-04 02:50 +0200
        Re: [RFC][PATCH] spin loop arch primitives for busy waiting Nicholas Piggin <npiggin@gmail.com> - 2017-04-04 05:10 +0200
          Re: [RFC][PATCH] spin loop arch primitives for busy waiting Nicholas Piggin <npiggin@gmail.com> - 2017-04-04 06:20 +0200
          Re: [RFC][PATCH] spin loop arch primitives for busy waiting David Miller <davem@davemloft.net> - 2017-04-05 16:10 +0200
            Re: [RFC][PATCH] spin loop arch primitives for busy waiting Nicholas Piggin <npiggin@gmail.com> - 2017-04-06 03:10 +0200
              Re: [RFC][PATCH] spin loop arch primitives for busy waiting Will Deacon <will.deacon@arm.com> - 2017-04-06 16:20 +0200
                Re: [RFC][PATCH] spin loop arch primitives for busy waiting Linus Torvalds <torvalds@linux-foundation.org> - 2017-04-06 17:20 +0200
                Re: [RFC][PATCH] spin loop arch primitives for busy waiting Peter Zijlstra <peterz@infradead.org> - 2017-04-06 18:40 +0200
                Re: [RFC][PATCH] spin loop arch primitives for busy waiting Linus Torvalds <torvalds@linux-foundation.org> - 2017-04-06 19:40 +0200
                Re: [RFC][PATCH] spin loop arch primitives for busy waiting Peter Zijlstra <peterz@infradead.org> - 2017-04-06 21:30 +0200
                Re: [RFC][PATCH] spin loop arch primitives for busy waiting Linus Torvalds <torvalds@linux-foundation.org> - 2017-04-06 21:50 +0200
                Re: [RFC][PATCH] spin loop arch primitives for busy waiting Nicholas Piggin <npiggin@gmail.com> - 2017-04-07 05:40 +0200
                Re: [RFC][PATCH] spin loop arch primitives for busy waiting Peter Zijlstra <peterz@infradead.org> - 2017-04-07 11:50 +0200
                Re: [RFC][PATCH] spin loop arch primitives for busy waiting Nicholas Piggin <npiggin@gmail.com> - 2017-04-07 13:30 +0200
                Re: [RFC][PATCH] spin loop arch primitives for busy waiting Nicholas Piggin <npiggin@gmail.com> - 2017-04-06 17:40 +0200
                Re: [RFC][PATCH] spin loop arch primitives for busy waiting Will Deacon <will.deacon@arm.com> - 2017-04-07 18:20 +0200

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