Re: [PATCH] blk-mq: only run the hardware queue if IO is pending

[Jens Axboe <axboe@kernel.dk>]

On 11/10/2017 10:29 AM, Christoph Hellwig wrote:
> On Fri, Nov 10, 2017 at 09:12:18AM -0700, Jens Axboe wrote:
>> Currently we are inconsistent in when we decide to run the queue. Using
>> blk_mq_run_hw_queues() we check if the hctx has pending IO before
>> running it, but we don't do that from the individual queue run function,
>> blk_mq_run_hw_queue(). This results in a lot of extra and pointless
>> queue runs, potentially, on flush requests and (much worse) on tag
>> starvation situations. This is observable just looking at the top
>> output, with lots of kworkers active. For the !async runs, it just adds
>> to the CPU overhead of blk-mq.
>>
>> Move the has-pending check into the run function instead of having
>> callers do it.
>>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> Do we even still need the blk_mq_hctx_has_pending helper at all?

We do, otherwise we end up running the queue for cases where we don't
need to. The caller doesn't always know if it's necessary. And even
if the caller can figure it out, it'd add complicated logic for some
of those cases (like flush). It's easier/cleaner and more efficient
to keep it.

-- 
Jens Axboe