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

[Jens Axboe]

On 05/04/2015 02:20 PM, Shaohua Li wrote:
> 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.

Agree, I will apply this one. Thanks!

--
Jens Axboe

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