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

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Date:   Wed, 22 May 2019 16:50:30 -0400
From:   Waiman Long <longman@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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 5/22/19 3:54 PM, Linus Torvalds wrote:
> 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.

The condition is to use non-underscore version only when

1) It is a preempt kernel; AND
2) It either have CONFIG_DEBUG_PREEMPT on, OR it is a non-x86 system.


> 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.

I was thinking of doing that originally, but then change it so x86
preempt kernel will also use the underscore version as long as
CONFIG_DEBUG_PREEMPT is not set.

I can change it back if that makes it less confusing.

Cheers,
Longman