[dm-devel] [PATCH v2 4/6] block: switch to per-cpu in-flight counters

[Mike Snitzer]

On Wed, Dec 05 2018 at  1:04pm -0500,
Jens Axboe <axboe at kernel.dk> wrote:

> On 12/5/18 11:03 AM, Mike Snitzer wrote:
> > On Wed, Dec 05 2018 at 12:54pm -0500,
> > Jens Axboe <axboe at kernel.dk> wrote:
> > 
> >> On 12/5/18 10:49 AM, Mike Snitzer wrote:
> >>> On Wed, Dec 05 2018 at 12:30pm -0500,
> >>> Jens Axboe <axboe at kernel.dk> wrote:
> >>>
> >>>> There's also no need to pass in the cpu, if we're not running with
> >>>> preempt disabled already we have a problem. 
> >>>
> >>> Why should this be any different than the part_stat_* interfaces?
> >>> __part_stat_add(), part_stat_read(), etc also use
> >>> per_cpu_ptr((part)->dkstats, (cpu) accessors.
> >>
> >> Maybe audit which ones actually need it? To answer the specific question,
> >> it's silly to pass in the cpu, if we're pinned already. That's true
> >> both programatically, but also for someone reading the code.
> > 
> > I understand you'd like to avoid excess interface baggage.  But seems to
> > me we'd be better off being consistent, when extending the percpu
> > portion of block core stats, and then do an incremental to clean it all
> > up.
> 
> The incremental should be done first in that case, it'd be silly to
> introduce something only to do a cleanup right after.

OK, all existing code for these percpu stats should follow the pattern:

  int cpu = part_stat_lock();

  <do percpu diskstats stuff>

  part_stat_unlock();

part_stat_lock() calls get_cpu() which does preempt_disable().  So to
your point: yes we have preempt disabled.  And yes we _could_ just use
smp_processor_id() in callers rather than pass 'cpu' to them.

Is that what you want to see?

Mike