Re: [PATCH 0/3] Introduce a light-weight queue close feature

[jianchao.wang]

Hi Ming

On 09/06/2018 05:27 AM, Ming Lei wrote:
> On Wed, Sep 05, 2018 at 12:09:43PM +0800, Jianchao Wang wrote:
>> Dear all
>>
>> As we know, queue freeze is used to stop new IO comming in and drain
>> the request queue. And the draining queue here is necessary, because
>> queue freeze kills the percpu-ref q_usage_counter and need to drain
>> the q_usage_counter before switch it back to percpu mode. This could
>> be a trouble when we just want to prevent new IO.
>>
>> In nvme-pci, nvme_dev_disable freezes queues to prevent new IO.
>> nvme_reset_work will unfreeze and wait to drain the queues. However,
>> if IO timeout at the moment, no body could do recovery as nvme_reset_work
>> is waiting. We will encounter IO hang.
> 
> As we discussed this nvme time issue before, I have pointed out that
> this is because of blk_mq_unfreeze_queue()'s limit which requires that
> unfreeze can only be done when this queue ref counter drops to zero.
> 
> For this nvme timeout case, we may relax the limit, for example,
> introducing another API of blk_freeze_queue_stop() as counter-pair of
> blk_freeze_queue_start(), and simply switch the percpu-ref to percpu mode
> from atomic mode inside the new API.

Looks like we cannot switch a percpu-ref to percpu mode directly w/o drain it.
Some references maybe lost.

static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
{
	unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
	int cpu;

	BUG_ON(!percpu_count);

	if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC))
		return;

	atomic_long_add(PERCPU_COUNT_BIAS, &ref->count);

	/*
	 * Restore per-cpu operation.  smp_store_release() is paired
	 * with READ_ONCE() in __ref_is_percpu() and guarantees that the
	 * zeroing is visible to all percpu accesses which can see the
	 * following __PERCPU_REF_ATOMIC clearing.
	 */
	for_each_possible_cpu(cpu)
		*per_cpu_ptr(percpu_count, cpu) = 0;

	smp_store_release(&ref->percpu_count_ptr,
			  ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC);
}

> 
>>
>> So introduce a light-weight queue close feature in this patch set
>> which could prevent new IO and needn't drain the queue.
> 
> Frankly speaking, IMO, it may not be an good idea to mess up the fast path
> just for handling the extremely unusual timeout event. The same is true
> for doing the preemp only stuff, as you saw I have posted patchset for
> killing it.
> 

In normal case, it is just a judgment like 

	if (unlikely(READ_ONCE(q->queue_gate))

It should not be a big deal.

Thanks
Jianchao

> Thanks,
> Ming
> 
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