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

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Message-ID: <20190524173915.GB9120@fuggles.cambridge.arm.com>
Date:   Fri, 24 May 2019 18:39:15 +0100
From:   Will Deacon <will.deacon@....com>
To:     Waiman Long <longman@...hat.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 Fri, May 24, 2019 at 01:35:39PM -0400, Waiman Long wrote:
> On 5/24/19 1:27 PM, Linus Torvalds wrote:
> > 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.
> 
> 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.

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.

Will