linux-kernel - Re: [PATCH v3] locking/lock_events: Use this_cpu

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Message-ID: <0f8883ee-6e31-a1dd-a749-6d2ba07e758d@redhat.com>
Date:   Fri, 24 May 2019 15:05:18 -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 v3] locking/lock_events: Use this_cpu_add() when necessary

On 5/24/19 3:00 PM, Linus Torvalds wrote:
> On Fri, May 24, 2019 at 11:54 AM Waiman Long <longman@...hat.com> wrote:
>>  v2: Simplify the condition to just preempt or !preempt.
>>  v3: Document the imprecise nature of the percpu count.
> My point was that if they are imprecise., then you shouldn't use CONFIG_PREEMPT.
>
> Because CONFIG_PREEMPT doesn't matter, and the count is imprecise with
> it or without it.
>
> So if they are imprecise, then what matters isn't whether the
> operation is atomic or not, and the real issue is avout whether it
> causes that "BUG: using __this_cpu_add() in preemptible" message.
>
> IOW, you should use the config option that matters and is relevant,
> namely CONFIG_DEBUG_PREEMPT.

Yes, that makes sense. I will update the patch again.

Thanks,
Longman