On 02/06/2017 12:53 PM, Omar Sandoval wrote:
> On Mon, Feb 06, 2017 at 12:39:57PM -0700, Jens Axboe wrote:
>> On 02/06/2017 12:24 PM, Omar Sandoval wrote:
>>> From: Omar Sandoval <osandov@xxxxxx>
>>>
>>> In blk_mq_sched_dispatch_requests(), we call blk_mq_sched_mark_restart()
>>> after we dispatch requests left over on our hardware queue dispatch
>>> list. This is so we'll go back and dispatch requests from the scheduler.
>>> In this case, it's only necessary to restart the hardware queue that we
>>> are running; there's no reason to run other hardware queues just because
>>> we are using shared tags.
>>>
>>> So, split out blk_mq_sched_mark_restart() into two operations, one for
>>> just the hardware queue and one for the whole request queue. The core
>>> code uses both, and I/O schedulers may also want to use them.
>>>
>>> Signed-off-by: Omar Sandoval <osandov@xxxxxx>
>>> ---
>>> Patch based on block/for-next.
>>>
>>> block/blk-mq-sched.c | 2 +-
>>> block/blk-mq-sched.h | 25 ++++++++++++++++++-------
>>> block/blk-mq.c | 5 ++++-
>>> 3 files changed, 23 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
>>> index ee455e7cf9d8..7538565359ea 100644
>>> --- a/block/blk-mq-sched.c
>>> +++ b/block/blk-mq-sched.c
>>> @@ -201,7 +201,7 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
>>> * needing a restart in that case.
>>> */
>>> if (!list_empty(&rq_list)) {
>>> - blk_mq_sched_mark_restart(hctx);
>>> + blk_mq_sched_mark_restart_hctx(hctx);
>>> blk_mq_dispatch_rq_list(hctx, &rq_list);
>>
>> What if we dispatched nothing on this hardware queue, and it currently
>> doesn't have any IO pending?
>
> Hm, so there are two ways that could happen. If it's because
> ->queue_rq() returned BLK_MQ_RQ_QUEUE_BUSY, then the driver is supposed
> to kick I/O off again, right?
>
> If it's because we failed to get a driver tag, then we'll call
> blk_mq_sched_mark_restart_queue() in the shared case. I just realized
> that there's a bug there, though. Since we already set the hctx restart
> bit, we won't set the queue restart bit. The below should work, and
> makes more sense in general.
>
> Or were you thinking of something else?
No, I think that covers it, I had not read far enough either to see that
you handle the shared tag case for tag starvation in the caller.
--
Jens Axboe
