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

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Message-ID: <20170406105958.196c6977@roar.ozlabs.ibm.com>
Date:   Thu, 6 Apr 2017 10:59:58 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     David Miller <davem@...emloft.net>
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

On Wed, 05 Apr 2017 07:01:57 -0700 (PDT)
David Miller <davem@...emloft.net> wrote:

> 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.
> 

Thanks for taking a look. The default spin primitives should just
continue to do the right thing for you in that case.

Arm has a yield instruction, ia64 has a pause... No unusual
requirements that I can see.

If there are no objections, I'll send the arch-independent part of
this through the powerpc tree (the last one I sent, which follows
Linus' preferred pattern).

Thanks,
Nick