linux-kernel - Re: [PATCH v2] locking/lock_events: Use this_cpu

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Date:   Fri, 24 May 2019 14:50:53 -0400
From:   Waiman Long <longman@...hat.com>
To:     Will Deacon <will.deacon@....com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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 v2] locking/lock_events: Use this_cpu_add() when necessary

On 5/24/19 2:32 PM, Will Deacon wrote:
> On Fri, May 24, 2019 at 02:11:23PM -0400, Waiman Long wrote:
>> On 5/24/19 1:39 PM, Will Deacon wrote:
>>
>>             And the whole "not precise" thing should be documented, of course.
>>
>>         Yes, I will update the patch to document that fact that the count may
>>         not be precise. Anyway even if we have a 1-2% error, it is not a big
>>         deal in term of presenting a global picture of what operations are being
>>         done.
>>
>>     I suppose one alternative would be to have a per-cpu local_t variable,
>>     and do the increments on that. However, that's probably worse than the
>>     current approach for x86.
>>
>> I don't quite understand what you mean by per-cpu local_t variable. A per-cpu
>> variable is either statically allocated or dynamically allocated. Even with
>> dynamical allocation, the same problem exists, I think unless you differentiate
>> between irq context and process context. That will make it a lot more messier,
>> I think.
> So I haven't actually tried this to see if it works, but all I meant was
> that you could replace the current:
>
> DECLARE_PER_CPU(unsigned long, lockevents[lockevent_num]);
>
> with:
>
> DECLARE_PER_CPU(local_t, lockevents[lockevent_num]);
>
> and then rework the inc/add macros to use a combination of raw_cpu_ptr
> and local_inc().
>
> I think that would allow you to get rid of the #ifdeffery, but it may
> introduce a small overhead for x86.

OK, I was not aware of the local_t type. Anyway, the x86 local_t type
perform similar single-instruction update. On other architectures that
can't do that, it will be a real atomic operation which will be more costly.

-Longman