linux-kernel - Re: [PATCH] locking/lock_events: Use this_cpu

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Date:   Wed, 22 May 2019 12:54:13 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH] locking/lock_events: Use this_cpu_add() when necessary

On Wed, May 22, 2019 at 8:40 AM Waiman Long <longman@...hat.com> wrote:
>
> +#if defined(CONFIG_PREEMPT) && \
> +   (defined(CONFIG_DEBUG_PREEMPT) || !defined(CONFIG_X86))
> +#define lockevent_percpu_inc(x)                this_cpu_inc(x)
> +#define lockevent_percpu_add(x, v)     this_cpu_add(x, v)

Why that CONFIG_X86 special case?

On x86, the regular non-underscore versionm is perfectly fine, and the
underscore is no faster or simpler.

So just make it be

   #if defined(CONFIG_PREEMPT)
     .. non-underscore versions..
   #else
     .. underscore versions ..
   #endif

and realize that x86 simply doesn't _care_. On x86, it will be one
single instruction regardless.

Non-x86 may prefer the underscore versions for the non-preempt case.

             Linus