On 5/24/19 2:32 PM, Will Deacon wrote: > 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.
OK, I was not aware of the local_t type. Anyway, the x86 local_t type perform similar single-instruction update. On other architectures that can't do that, it will be a real atomic operation which will be more costly. -Longman
