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

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Message-Id: <20170405.070157.871721909352646302.davem@davemloft.net>
Date:   Wed, 05 Apr 2017 07:01:57 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     npiggin@...il.com
Cc:     torvalds@...ux-foundation.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, anton@...ba.org,
        linuxppc-dev@...abs.org
Subject: Re: [RFC][PATCH] spin loop arch primitives for busy waiting

From: Nicholas Piggin <npiggin@...il.com>
Date: Tue, 4 Apr 2017 13:02:33 +1000

> On Mon, 3 Apr 2017 17:43:05 -0700
> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
>> But that depends on architectures having some pattern that we *can*
>> abstract. Would some "begin/in-loop/end" pattern like the above be
>> sufficient?
> 
> Yes. begin/in/end would be sufficient for powerpc SMT priority, and
> for x86, and it looks like sparc64 too. So we could do that if you
> prefer.

Sparc64 has two cases, on older chips we can induce a cpu thread yield
with a special sequence of instructions, and on newer chips we have
a bonafide pause instruction.

So cpu_relax() all by itself pretty much works for us.