Re: [PATCH -next 2/2] block, bfq: make bfq_has_work() more accurate

[yukuai (C)]

在 2022/05/13 1:10, Jan Kara 写道:
> On Thu 12-05-22 09:30:16, yukuai (C) wrote:
> > On 2022/05/11 22:08, Jan Kara wrote:
> > > On Tue 10-05-22 21:16:29, Yu Kuai wrote:
> > > > bfq_has_work() is using busy_queues currently, which is not accurate
> > > > because bfq_queue is busy doesn't represent that it has requests. Since
> > > > bfqd aready has a counter 'queued' to record how many requests are in
> > > > bfq, use it instead of busy_queues.
> > > >
> > > > Noted that bfq_has_work() can be called with 'bfqd->lock' held, thus the
> > > > lock can't be held in bfq_has_work() to protect 'bfqd->queued'.
> > > >
> > > > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> > >
> > > So did you find this causing any real problem? Because bfq queue is
> > > accounted among busy queues once bfq_add_bfqq_busy() is called. And that
> > > happens once a new request is inserted into the queue so it should be very
> > > similar to bfqd->queued.
> > >
> > > 								Honza
> >
> > Hi,
> >
> > The related problem is described here:
> >
> > https://lore.kernel.org/all/20220510112302.1215092-1-yukuai3@xxxxxxxxxx/
> >
> > The root cause of the panic is a linux-block problem, however, it can
> > be bypassed if bfq_has_work() is accurate. On the other hand,
> > unnecessary run_work will be triggered if bfqq stays busy:
> >
> > __blk_mq_run_hw_queue
> >   __blk_mq_sched_dispatch_requests
> >    __blk_mq_do_dispatch_sched
> >     if (!bfq_has_work())
> >      break;
> >     blk_mq_delay_run_hw_queues -> run again after 3ms
>
> Ah, I see. So it is the other way around than I thought. Due to idling
> bfq_tot_busy_queues() can be greater than 0 even if there are no requests
> to dispatch. Indeed. OK, the patch makes sense. But please use WRITE_ONCE
> for the updates of bfqd->queued. Otherwise the READ_ONCE does not really
> make sense (it can still result in some bogus value due to compiler
> optimizations on the write side).

Thanks for you adivce, I'll send a new version.

Kuai