Messages in this thread | | | Date | Fri, 24 May 2019 19:32:58 +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 02:11:23PM -0400, Waiman Long wrote: > On 5/24/19 1:39 PM, Will Deacon wrote: > > 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. > > I don't quite understand what you mean by per-cpu local_t variable. A per-cpu > variable is either statically allocated or dynamically allocated. Even with > dynamical allocation, the same problem exists, I think unless you differentiate > between irq context and process context. That will make it a lot more messier, > I think.
So I haven't actually tried this to see if it works, but all I meant was that you could replace the current:
DECLARE_PER_CPU(unsigned long, lockevents[lockevent_num]);
with:
DECLARE_PER_CPU(local_t, lockevents[lockevent_num]);
and then rework the inc/add macros to use a combination of raw_cpu_ptr and local_inc().
I think that would allow you to get rid of the #ifdeffery, but it may introduce a small overhead for x86.
Will
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