Re: [PATCH V5 3/4] blk-mq: clear stale request in tags->rq[] before freeing one request pool

[Ming Lei <ming.lei@redhat.com>]

On Thu, May 06, 2021 at 08:16:25PM -0700, Bart Van Assche wrote:
> On 5/6/21 7:05 PM, Ming Lei wrote:
> > Putting the lock pair after clearing rq mapping should work, but not see
> > any benefit: not very readable, and memory barrier knowledge is required for
> > understanding its correctness(cmpxchg has to be completed before unlock), ...,
> > so is it better idea to move lock pair after clearing rq mapping?
> 
> It depends on how much time will be spent inside
> blk_mq_clear_rq_mapping(). If the time spent in the nested loop in
> blk_mq_clear_rq_mapping() would be significant then the proposed change
> will help to reduce interrupt latency in blk_mq_find_and_get_req().

interrupt latency in blk_mq_find_and_get_req() shouldn't be increased
because interrupt won't be disabled when spinning on the lock. But interrupt
may be disabled for a while in blk_mq_clear_rq_mapping() in case of big
nr_requests and hw queue depth.

Fair enough, will take this way for not holding lock for too long.

Thanks, 
Ming