Re: [PATCH] blk-mq: don't lose requests if a stopped queue restarts

[Jens Axboe <axboe@fb.com>]

On 05/04/2015 02:20 PM, Shaohua Li wrote:
> On Mon, May 04, 2015 at 01:56:42PM -0600, Jens Axboe wrote:
>> On 05/04/2015 01:51 PM, Shaohua Li wrote:
>>> On Mon, May 04, 2015 at 01:17:19PM -0600, Jens Axboe wrote:
>>>> On 05/02/2015 06:31 PM, Shaohua Li wrote:
>>>>> Normally if driver is busy to dispatch a request the logic is like below:
>>>>> block layer:					driver:
>>>>> 	__blk_mq_run_hw_queue
>>>>> a.						blk_mq_stop_hw_queue
>>>>> b.	rq add to ctx->dispatch
>>>>>
>>>>> later:
>>>>> 1.						blk_mq_start_hw_queue
>>>>> 2.	__blk_mq_run_hw_queue
>>>>>
>>>>> But it's possible step 1-2 runs between a and b. And since rq isn't in
>>>>> ctx->dispatch yet, step 2 will not run rq. The rq might get lost if
>>>>> there are no subsequent requests kick in.
>>>>
>>>> Good catch! But the patch introduces a potentially never ending loop
>>>> in __blk_mq_run_hw_queue(). Not sure how we can fully close it, but
>>>> it might be better to punt the re-run after adding the requests back
>>>> to the worker. That would turn a potential busy loop (until requests
>>>> complete) into something with nicer behavior, at least. Ala
>>>>
>>>> if (!test_bit(BLK_MQ_S_STOPPED, &hctx->state))
>>>>       kblockd_schedule_delayed_work_on(blk_mq_hctx_next_cpu(hctx),
>>>>                                          &hctx->run_work, 0);
>>>
>>> My first version of the patch is like this, but I changed my mind later.
>>> The assumption is driver will stop queue if it's busy to dispatch
>>> request.  If the driver is buggy, we will have the endless loop here.
>>> Should we assume drivers will not do the right thing?
>>
>> There's really no contract that says the driver MUST stop the queue
>> for busy. It could, legitimately, decide to just always run the
>> queue when requests complete.
>>
>> It might be better to simply force this behavior. If we get a BUSY,
>> stop the queue from __blk_mq_run_hw_queue(). And if the bit isn't
>> still set on re-add, then we know we need to re-run it. I think that
>> would be a cleaner API, less fragile, and harder to get wrong. The
>> down side is that now this stop happens implicitly by the core, and
>> the driver must now have an asymmetric queue start when it frees the
>> limited resource that caused the BUSY return. Either that, or we
>> define a 2nd set of start/stop bits, one used exclusively by the
>> driver and one used exclusively by blk-mq. Then blk-mq could restart
>> the queue on completion of a request, since it would then know that
>> blk-mq was the one that stopped it.
>
> Agree. I'll make the rerun async for now and leave above as a future
> improvement.

Agree, I will apply this one. Thanks!

-- 
Jens Axboe