> On Aug 26, 2024, at 15:06, Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
>
> On Fri, Aug 23, 2024 at 7:28 PM Ming Lei <ming.lei@xxxxxxxxxx> 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
>
> So I think fixing blk_mq_run_hw_queue() could cover all of the situations.
> Maybe I thought wrongly. Please correct me.
>
> Muchun,
> Thanks.
