Re: [PATCH 4/4] block: fix fix ordering between checking QUEUE_FLAG_QUIESCED and adding requests to hctx->dispatch

[Muchun Song <muchun.song@linux.dev>]

> On Aug 27, 2024, at 15:24, Muchun Song <muchun.song@linux.dev> wrote:
> 
> 
> 
>> On Aug 26, 2024, at 17:20, Ming Lei <ming.lei@redhat.com> wrote:
>> 
>> On Mon, Aug 26, 2024 at 03:33:18PM +0800, Muchun Song wrote:
>>> 
>>> 
>>>> On Aug 26, 2024, at 15:06, Muchun Song <songmuchun@bytedance.com> wrote:
>>>> 
>>>> On Fri, Aug 23, 2024 at 7:28 PM Ming Lei <ming.lei@redhat.com> wrote:
>>>>> 
>>>>> On Sun, Aug 11, 2024 at 06:19:21 PM +0800, Muchun Song wrote:
>>>>>> Supposing the following scenario.
>>>>>> 
>>>>>> CPU0                                                                CPU1
>>>>>> 
>>>>>> blk_mq_request_issue_directly()                                     blk_mq_unquiesce_queue()
>>>>>>  if (blk_queue_quiesced())                                           blk_queue_flag_clear(QUEUE_FLAG_QUIESCED)   3) store
>>>>>>      blk_mq_insert_request()                                         blk_mq_run_hw_queues()
>>>>>>          /*                                                              blk_mq_run_hw_queue()
>>>>>>           * Add request to dispatch list or set bitmap of                    if (!blk_mq_hctx_has_pending())     4) load
>>>>>>           * software queue.                  1) store                            return
>>>>>>           */
>>>>>>      blk_mq_run_hw_queue()
>>>>>>          if (blk_queue_quiesced())           2) load
>>>>>>              return
>>>>>>          blk_mq_sched_dispatch_requests()
>>>>>> 
>>>>>> The full memory barrier should be inserted between 1) and 2), as well as
>>>>>> between 3) and 4) to make sure that either CPU0 sees QUEUE_FLAG_QUIESCED is
>>>>>> cleared or CPU1 sees dispatch list or setting of bitmap of software queue.
>>>>>> Otherwise, either CPU will not re-run the hardware queue causing starvation.
>>>>> 
>>>>> Memory barrier shouldn't serve as bug fix for two slow code paths.
>>>>> 
>>>>> One simple fix is to add helper of blk_queue_quiesced_lock(), and
>>>>> call the following check on CPU0:
>>>>> 
>>>>>      if (blk_queue_quiesced_lock())
>>>>>       blk_mq_run_hw_queue();
>>>> 
>>>> This only fixes blk_mq_request_issue_directly(), I think anywhere that
>>>> matching this
>>>> pattern (inserting a request to dispatch list and then running the
>>>> hardware queue)
>>>> should be fixed. And I think there are many places which match this
>>>> pattern (E.g.
>>>> blk_mq_submit_bio()). The above graph should be adjusted to the following.
>>>> 
>>>> CPU0                                        CPU1
>>>> 
>>>> blk_mq_insert_request()         1) store    blk_mq_unquiesce_queue()
>>>> blk_mq_run_hw_queue()
>>>> blk_queue_flag_clear(QUEUE_FLAG_QUIESCED)       3) store
>>>>  if (blk_queue_quiesced())   2) load         blk_mq_run_hw_queues()
>>>>      return                                      blk_mq_run_hw_queue()
>>>>  blk_mq_sched_dispatch_requests()                    if
>>>> (!blk_mq_hctx_has_pending())     4) load
>>>>                                                          return
>>> 
>>> Sorry. There is something wrong with my email client. Resend the graph.
>>> 
>>> CPU0                                        CPU1
>>> 
>>> blk_mq_insert_request()         1) store    blk_mq_unquiesce_queue()
>>> blk_mq_run_hw_queue()                       blk_queue_flag_clear(QUEUE_FLAG_QUIESCED)       3) store
>>>   if (blk_queue_quiesced())   2) load         blk_mq_run_hw_queues()
>>>       return                                      blk_mq_run_hw_queue()
>>>   blk_mq_sched_dispatch_requests()                    if (!blk_mq_hctx_has_pending())     4) load
>>>                                                           return
>> 
>> OK.
>> 
>> The issue shouldn't exist if blk_queue_quiesced() return false in
>> blk_mq_run_hw_queue(), so it is still one race in two slow paths?
>> 
>> I guess the barrier-less approach should work too, such as:
>> 
> 
> If we prefer barrier-less approach, I think the following solution
> will work as well, I'll use it in v2. Thanks.
> 
>> 
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index e3c3c0c21b55..632261982a77 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -2202,6 +2202,12 @@ void blk_mq_delay_run_hw_queue(struct blk_mq_hw_ctx *hctx, unsigned long msecs)
>> }
>> EXPORT_SYMBOL(blk_mq_delay_run_hw_queue);
>> 
>> +static inline bool blk_mq_hw_queue_need_run(struct blk_mq_hw_ctx *hctx)
>> +{
>> + 	return !blk_queue_quiesced(hctx->queue) &&
>> + 		blk_mq_hctx_has_pending(hctx);
>> +}
>> +
>> /**
>> * blk_mq_run_hw_queue - Start to run a hardware queue.
>> * @hctx: Pointer to the hardware queue to run.
>> @@ -2231,11 +2237,19 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
>> * quiesced.
>> */
>> 	__blk_mq_run_dispatch_ops(hctx->queue, false,
>> - 		need_run = !blk_queue_quiesced(hctx->queue) &&
>> - 		blk_mq_hctx_has_pending(hctx));
>> + 		need_run = blk_mq_hw_queue_need_run(hctx));
>> 
>> - 	if (!need_run)
>> - 		return;
>> + 	if (!need_run) {
>> + 		unsigned long flags;
>> +
>> + 		/* sync with unquiesce */
>> + 		spin_lock_irqsave(&hctx->queue->queue_lock, flags);

After some time thought, I think here we need a big comment to explain
why we need to sync. Because there are other caller of blk_queue_quiesced()
which do not need to hold ->queue_lock to sync. Then, I am thinking
is ->queue_lock really easier to be maintained than mb? For developers,
we still need to care about this, right? I don't see any obvious benefit.
And the mb approach seems more efficient than spinlock. Something like:

	if (!need_run) {
		/* Add a comment here to explain what's going on here. */
		smp_mb();
		need_run = blk_mq_hw_queue_need_run(hctx);
		if (!need_run)
			return;
	}

I am not objecting to your approach, I want to know if you insist on
barrier-less approach here. If yes, I'm fine with this approach. I can
use it in v2.

Muhcun,
Thanks.

>> + 		need_run = blk_mq_hw_queue_need_run(hctx);
> 
> One question here: should we use __blk_mq_run_dispatch_ops()? I saw a comment above.
> It seems it is safe to call blk_mq_hw_queue_need_run under [s]rcu lock.
> 
> Thanks.
> 
>> + 		spin_unlock_irqrestore(&hctx->queue->queue_lock, flags);
>> +
>> + 		if (!need_run)
>> + 			return;
>> + 	}
>> 
>> 	if (async || !cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask)) {
>> 		blk_mq_delay_run_hw_queue(hctx, 0);
>> 
>> 
>> thanks,
>> Ming
> 
>