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Subject: Re: [PATCH v2] locking/lock_events: Use this_cpu_add() when necessary
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Will Deacon <will.deacon@arm.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        huang ying <huang.ying.caritas@gmail.com>
References: <20190524165346.26373-1-longman@redhat.com>
 <20190524171939.GA9120@fuggles.cambridge.arm.com>
 <CAHk-=wiQ3kbk1G40ofSMu7qGhrX4PgngN64jGnttOcNCvKy6EA@mail.gmail.com>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <8ceebb1c-e8f1-8bc5-e032-48f1a653a979@redhat.com>
Date:   Fri, 24 May 2019 13:35:39 -0400
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On 5/24/19 1:27 PM, Linus Torvalds wrote:
> On Fri, May 24, 2019 at 10:19 AM Will Deacon <will.deacon@arm.com> wrote:
>> Are you sure this works wrt IRQs? For example, if I take an interrupt when
>> trying to update the counter, and then the irq handler takes a qspinlock
>> which in turn tries to update the counter. Would I lose an update in that
>> scenario?
> Sounds about right.
>
> We might decide that the lock event counters are not necessarily
> precise, but just rough guide-line statistics ("close enough in
> practice")
>
> But that would imply that it shouldn't be dependent on CONFIG_PREEMPT
> at all, and we should always use the double-underscore version, except
> without the debug checking.
>
> Maybe the #ifdef should just be CONFIG_PREEMPT_DEBUG, with a comment
> saying "we're not exact, but debugging complains, so if you enable
> debugging it will be slower and precise". Because I don't think we
> have a "do this unsafely and without any debugging" option.

I am not too worry about losing count here and there once in a while
because of interrupts, but the possibility of having the count from one
CPU to be put into another CPU in a preempt kernel may distort the total
count significantly. This is what I want to avoid.


>
> 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.

Cheers,
Longman