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 13:22:52 +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:

> 
> * 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.

I mean, this third patch of yours:

   [PATCH] x86/percpu: Define {raw,this}_cpu_try_cmpxchg{64,128}

Had a proper disassembly comparison - so I've applied all 3 optimization 
patches to tip:x86/asm as:

  b8e3dfa16ec5 ("x86/percpu: Use raw_cpu_try_cmpxchg() in preempt_count_set()")
  5f863897d964 ("x86/percpu: Define raw_cpu_try_cmpxchg and this_cpu_try_cmpxchg()")
  54cd971c6f44 ("x86/percpu: Define {raw,this}_cpu_try_cmpxchg{64,128}")

Thanks,

	Ingo