On 5/24/19 1:27 PM, Linus Torvalds wrote: > On Fri, May 24, 2019 at 10:19 AM Will Deacon <[email protected]> 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
