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

[Jens Axboe <axboe@kernel.dk>]

On 12/5/18 11:18 AM, Mike Snitzer wrote:
> On Wed, Dec 05 2018 at  1:04pm -0500,
> Jens Axboe <axboe@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@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@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?

Something like that, yes.

-- 
Jens Axboe