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). Honza > > > --- > > > block/bfq-iosched.c | 4 ++-- > > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > > > diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c > > > index 61750696e87f..1d2f8110c26b 100644 > > > --- a/block/bfq-iosched.c > > > +++ b/block/bfq-iosched.c > > > @@ -5063,11 +5063,11 @@ static bool bfq_has_work(struct blk_mq_hw_ctx *hctx) > > > struct bfq_data *bfqd = hctx->queue->elevator->elevator_data; > > > /* > > > - * Avoiding lock: a race on bfqd->busy_queues should cause at > > > + * Avoiding lock: a race on bfqd->queued should cause at > > > * most a call to dispatch for nothing > > > */ > > > return !list_empty_careful(&bfqd->dispatch) || > > > - bfq_tot_busy_queues(bfqd) > 0; > > > + READ_ONCE(bfqd->queued); > > > } > > > static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx) > > > -- > > > 2.31.1 > > > -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
