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

[Will Deacon]

On Wed, May 22, 2019 at 12:54:13PM -0700, Linus Torvalds wrote:
> On Wed, May 22, 2019 at 8:40 AM Waiman Long <longman@xxxxxxxxxx> wrote:
> >
> > +#if defined(CONFIG_PREEMPT) && \
> > +   (defined(CONFIG_DEBUG_PREEMPT) || !defined(CONFIG_X86))
> > +#define lockevent_percpu_inc(x)                this_cpu_inc(x)
> > +#define lockevent_percpu_add(x, v)     this_cpu_add(x, v)
> 
> Why that CONFIG_X86 special case?
> 
> On x86, the regular non-underscore versionm is perfectly fine, and the
> underscore is no faster or simpler.
> 
> So just make it be
> 
>    #if defined(CONFIG_PREEMPT)
>      .. non-underscore versions..
>    #else
>      .. underscore versions ..
>    #endif
> 
> and realize that x86 simply doesn't _care_. On x86, it will be one
> single instruction regardless.
> 
> Non-x86 may prefer the underscore versions for the non-preempt case.

To be honest, given this depends on LOCK_EVENT_COUNTS, I'd be inclined to
keep things simple and drop the underscore versions entirely. Saves having
to worry about things like "could I take an interrupt during the add?".

Will