Re: [PATCH] blk-mq: don't lose requests if a stopped queue restarts

[Shaohua Li]

On Mon, May 04, 2015 at 01:56:42PM -0600, Jens Axboe wrote:
> On 05/04/2015 01:51 PM, Shaohua Li wrote:
> >On Mon, May 04, 2015 at 01:17:19PM -0600, Jens Axboe wrote:
> >>On 05/02/2015 06:31 PM, Shaohua Li wrote:
> >>>Normally if driver is busy to dispatch a request the logic is like below:
> >>>block layer:					driver:
> >>>	__blk_mq_run_hw_queue
> >>>a.						blk_mq_stop_hw_queue
> >>>b.	rq add to ctx->dispatch
> >>>
> >>>later:
> >>>1.						blk_mq_start_hw_queue
> >>>2.	__blk_mq_run_hw_queue
> >>>
> >>>But it's possible step 1-2 runs between a and b. And since rq isn't in
> >>>ctx->dispatch yet, step 2 will not run rq. The rq might get lost if
> >>>there are no subsequent requests kick in.
> >>
> >>Good catch! But the patch introduces a potentially never ending loop
> >>in __blk_mq_run_hw_queue(). Not sure how we can fully close it, but
> >>it might be better to punt the re-run after adding the requests back
> >>to the worker. That would turn a potential busy loop (until requests
> >>complete) into something with nicer behavior, at least. Ala
> >>
> >>if (!test_bit(BLK_MQ_S_STOPPED, &hctx->state))
> >>      kblockd_schedule_delayed_work_on(blk_mq_hctx_next_cpu(hctx),
> >>                                         &hctx->run_work, 0);
> >
> >My first version of the patch is like this, but I changed my mind later.
> >The assumption is driver will stop queue if it's busy to dispatch
> >request.  If the driver is buggy, we will have the endless loop here.
> >Should we assume drivers will not do the right thing?
> 
> There's really no contract that says the driver MUST stop the queue
> for busy. It could, legitimately, decide to just always run the
> queue when requests complete.
> 
> It might be better to simply force this behavior. If we get a BUSY,
> stop the queue from __blk_mq_run_hw_queue(). And if the bit isn't
> still set on re-add, then we know we need to re-run it. I think that
> would be a cleaner API, less fragile, and harder to get wrong. The
> down side is that now this stop happens implicitly by the core, and
> the driver must now have an asymmetric queue start when it frees the
> limited resource that caused the BUSY return. Either that, or we
> define a 2nd set of start/stop bits, one used exclusively by the
> driver and one used exclusively by blk-mq. Then blk-mq could restart
> the queue on completion of a request, since it would then know that
> blk-mq was the one that stopped it.

Agree. I'll make the rerun async for now and leave above as a future
improvement.