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

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Message-ID: <e6f9465a-2d26-23c4-96b6-4c5945993e6d@redhat.com>
Date:   Fri, 24 May 2019 13:00:14 -0400
From:   Waiman Long <longman@...hat.com>
To:     Will Deacon <will.deacon@....com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.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 5/23/19 10:58 AM, Will Deacon wrote:
> On Wed, May 22, 2019 at 12:54:13PM -0700, 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.
>>
>> 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.
> To be honest, given this depends on LOCK_EVENT_COUNTS, I'd be inclined to
> keep things simple and drop the underscore versions entirely. Saves having
> to worry about things like "could I take an interrupt during the add?".
>
I have sent out the v2 patch that simplifies the condition. Now the
underscore versions will be used for !preempt kernel and non-underscore
version used in preempt kernel. The non-underscore versions may generate
a lot more unnecessary code when CONFIG_PREEMPT_COUNT is defined.

Cheers,
Longman