Messages in this thread | | | Date | Fri, 24 May 2019 18:39:15 +0100 | From | Will Deacon <> | Subject | Re: [PATCH v2] locking/lock_events: Use this_cpu_add() when necessary |
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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@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.
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
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