Re: [PATCH v2] locking/lock_events: Use this_cpu_add() when necessary

[Will Deacon <will.deacon@arm.com>]

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