Re: [PATCH V6 4/5] blk-mq-sched: improve dispatching from sw queue

[Jens Axboe <axboe@kernel.dk>]

On 10/12/2017 09:22 AM, Ming Lei wrote:
> On Thu, Oct 12, 2017 at 08:52:12AM -0600, Jens Axboe wrote:
>> On 10/12/2017 04:01 AM, Ming Lei wrote:
>>> On Tue, Oct 10, 2017 at 11:23:45AM -0700, Omar Sandoval wrote:
>>>> On Mon, Oct 09, 2017 at 07:24:23PM +0800, Ming Lei wrote:
>>>>> SCSI devices use host-wide tagset, and the shared driver tag space is
>>>>> often quite big. Meantime there is also queue depth for each lun(
>>>>> .cmd_per_lun), which is often small, for example, on both lpfc and
>>>>> qla2xxx, .cmd_per_lun is just 3.
>>>>>
>>>>> So lots of requests may stay in sw queue, and we always flush all
>>>>> belonging to same hw queue and dispatch them all to driver, unfortunately
>>>>> it is easy to cause queue busy because of the small .cmd_per_lun.
>>>>> Once these requests are flushed out, they have to stay in hctx->dispatch,
>>>>> and no bio merge can participate into these requests, and sequential IO
>>>>> performance is hurt a lot.
>>>>>
>>>>> This patch introduces blk_mq_dequeue_from_ctx for dequeuing request from
>>>>> sw queue so that we can dispatch them in scheduler's way, then we can
>>>>> avoid to dequeue too many requests from sw queue when ->dispatch isn't
>>>>> flushed completely.
>>>>>
>>>>> This patch improves dispatching from sw queue when there is per-request-queue
>>>>> queue depth by taking request one by one from sw queue, just like the way
>>>>> of IO scheduler.
>>>>
>>>> This still didn't address Jens' concern about using q->queue_depth as
>>>> the heuristic for whether to do the full sw queue flush or one-by-one
>>>> dispatch. The EWMA approach is a bit too complex for now, can you please
>>>> try the heuristic of whether the driver ever returned BLK_STS_RESOURCE?
>>>
>>> That can be done easily, but I am not sure if it is good.
>>>
>>> For example, inside queue rq path of NVMe, kmalloc(GFP_ATOMIC) is
>>> often used, if kmalloc() returns NULL just once, BLK_STS_RESOURCE
>>> will be returned to blk-mq, then blk-mq will never do full sw
>>> queue flush even when kmalloc() always succeed from that time
>>> on.
>>
>> Have it be a bit more than a single bit, then. Reset it every x IOs or
>> something like that, that'll be more representative of transient busy
>> conditions anyway.
> 
> OK, that can be done via a simplified EWMA by considering
> the dispatch result only.

Yes, if it's kept simple enough, then that would be fine. I'm not totally
against EWMA, I just don't want to have any of this over-engineered.
Especially not when it's a pretty simple thing, we don't care about
averages, basically only if we ever see BLK_STS_RESOURCE in any kind
of recurring fashion.

-- 
Jens Axboe