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

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Date:   Fri, 24 May 2019 10:27:05 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Waiman Long <longman@...hat.com>,
        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 Fri, May 24, 2019 at 10:19 AM Will Deacon <will.deacon@....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.

And the whole "not precise" thing should be documented, of course.

I can't imagine that people would rely on _exact_ lock statistics, but
hey, there are a lot of things people do that I can't fathom, so
that's not necessarily a strong argument.

Comments?

                  Linus