linux-kernel - Re: [PATCH 2/2] x86/percpu: Use raw_cpu_try_cmpxchg in preempt_count

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Date:   Fri, 15 Sep 2023 13:15:35 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 2/2] x86/percpu: Use raw_cpu_try_cmpxchg in
 preempt_count_set


* Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Uros Bizjak <ubizjak@...il.com> wrote:
> 
> > Use raw_cpu_try_cmpxchg instead of raw_cpu_cmpxchg (*ptr, old, new) == old.
> > x86 CMPXCHG instruction returns success in ZF flag, so this change saves a
> > compare after cmpxchg (and related move instruction in front of cmpxchg).
> > 
> > Also, raw_cpu_try_cmpxchg implicitly assigns old *ptr value to "old" when
> > cmpxchg fails. There is no need to re-read the value in the loop.
> > 
> > No functional change intended.
> > 
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: Borislav Petkov <bp@...en8.de>
> > Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> > Cc: "H. Peter Anvin" <hpa@...or.com>
> > Signed-off-by: Uros Bizjak <ubizjak@...il.com>
> > ---
> >  arch/x86/include/asm/preempt.h | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/preempt.h b/arch/x86/include/asm/preempt.h
> > index 2d13f25b1bd8..4527e1430c6d 100644
> > --- a/arch/x86/include/asm/preempt.h
> > +++ b/arch/x86/include/asm/preempt.h
> > @@ -31,11 +31,11 @@ static __always_inline void preempt_count_set(int pc)
> >  {
> >  	int old, new;
> >  
> > +	old = raw_cpu_read_4(pcpu_hot.preempt_count);
> >  	do {
> > -		old = raw_cpu_read_4(pcpu_hot.preempt_count);
> >  		new = (old & PREEMPT_NEED_RESCHED) |
> >  			(pc & ~PREEMPT_NEED_RESCHED);
> > -	} while (raw_cpu_cmpxchg_4(pcpu_hot.preempt_count, old, new) != old);
> > +	} while (!raw_cpu_try_cmpxchg_4(pcpu_hot.preempt_count, &old, new));
> 
> It would be really nice to have a before/after comparison of generated 
> assembly code in the changelog, to demonstrate the effectiveness of this 
> optimization.

Never mind, you did exactly that in the September 6 variation of these 
changes. I'll apply those.

Thanks,

	Ingo