Re: [RFC PATCH] blk-mq: implement queue quiesce via percpu_ref for BLK_MQ_F_BLOCKING

Sagi Grimberg
2020-07-29T22:43:04Z

>>> void blk_mq_quiesce_queue(struct request_queue *q)
>>> {
>>> - struct blk_mq_hw_ctx *hctx;
>>> - unsigned int i;
>>> - bool rcu = false;
>>> -
>>> blk_mq_quiesce_queue_nowait(q);
>>>
>>> - queue_for_each_hw_ctx(q, hctx, i) {
>>> - if (hctx->flags & BLK_MQ_F_BLOCKING)
>>> - synchronize_srcu(hctx->srcu);
>>> - else
>>> - rcu = true;
>>> - }
>>> - if (rcu)
>>> + if (q->tag_set->flags & BLK_MQ_F_BLOCKING) {
>>> + percpu_ref_kill(&q->dispatch_counter);
>>> + wait_event(q->mq_quiesce_wq,
>>> + percpu_ref_is_zero(&q->dispatch_counter));
>>> + } else
>>> synchronize_rcu();
>>> }
>>
>>
>>
>>> +static void hctx_lock(struct blk_mq_hw_ctx *hctx)
>>> {
>>> - if (!(hctx->flags & BLK_MQ_F_BLOCKING)) {
>>> - /* shut up gcc false positive */
>>> - *srcu_idx = 0;
>>> + if (!(hctx->flags & BLK_MQ_F_BLOCKING))
>>> rcu_read_lock();
>>> - } else
>>> - *srcu_idx = srcu_read_lock(hctx->srcu);
>>> + else
>>> + percpu_ref_get(&hctx->queue->dispatch_counter);
>>> }
>>
>> percpu_ref_get() will always succeed, even after quiesce kills it.
>> Isn't it possible that 'percpu_ref_is_zero(&q->dispatch_counter))' may
>> never reach 0? We only need to ensure that dispatchers will observe
>> blk_queue_quiesced(). That doesn't require that there are no current
>> dispatchers.
>
> IMO it shouldn't be one issue in reality, because:
>
> - when dispatch can't make progress, the submission side will finally
> stop because we either run queue from submission side or request
> completion
>
> - submission side stops because we always have very limited requests
>
> - completion side stops because requests queued to device is limited
> too

I don't think that any requests should pass after the kill was called,
otherwise how can we safely quiesce if requests can come in after
it?

>
> We still can handle this case by not dispatch in case that percpu_ref_tryget()

You meant tryget_live right?

> returns false, which will change the usage into the following way:
>
> if (hctx_lock(hctx)) {
> blk_mq_sched_dispatch_requests(hctx);
> hctx_unlock(hctx);
> }
>
> And __blk_mq_try_issue_directly() needs a bit special treatment because
> the request has to be inserted to queue after queue becomes quiesced.

Agreed.