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 11:47:09 +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


* 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.

Thanks,

	Ingo