Re: [PATCH -next v2 2/2] block, bfq: make bfq_has_work() more accurate

[yukuai (C)]

在 2022/05/17 23:06, Paolo Valente 写道:
> > Il giorno 17 mag 2022, alle ore 16:21, Paolo Valente <paolo.valente@xxxxxxxxxx> ha scritto:
> >
> >
> >
> > > Il giorno 16 mag 2022, alle ore 11:56, Jan Kara <jack@xxxxxxx> ha scritto:
> > >
> > > On Fri 13-05-22 10:35:07, 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.
> >
> > The number of requests queued is not equal to the number of busy
> > queues (it is >=).
>
> No, sorry. It is actually != in general.
Hi, Paolo

I'm aware that number of requests queued is not equal to the number of
busy queues, and that is the motivation of this patch.

> In particular, if queued == 0 but there are busy queues (although
> still waiting for I/O to arrive), then responding that there is no
> work caused blk-mq to stop asking, and hence an I/O freeze.  IOW I/O
> eventually arrives for a busy queue, but blk-mq does not ask for a new
> request any longer.  But maybe things have changed around bfq since
> then.

The problem is that if queued == 0 while there are busy queues, is there
any point to return true in bfq_has_work() ? IMO, it will only cause
unecessary run queue. And if new request arrives,
blk_mq_sched_insert_request() will trigger a run queue.

Thanks,
Kuai
> Paolo
>
> >   If this patch is based on this assumption then
> > unfortunately it is wrong :(
> >
> > Paolo
> >
> > > > 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>
> > >
> > > Looks good. Feel free to add:
> > >
> > > Reviewed-by: Jan Kara <jack@xxxxxxx>
> > >
> > > 								Honza
> > >
> > > > ---
> > > > block/bfq-iosched.c | 16 ++++++++++++----
> > > > 1 file changed, 12 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> > > > index 61750696e87f..740dd83853a6 100644
> > > > --- a/block/bfq-iosched.c
> > > > +++ b/block/bfq-iosched.c
> > > > @@ -2210,7 +2210,11 @@ static void bfq_add_request(struct request *rq)
> > > >
> > > > 	bfq_log_bfqq(bfqd, bfqq, "add_request %d", rq_is_sync(rq));
> > > > 	bfqq->queued[rq_is_sync(rq)]++;
> > > > -	bfqd->queued++;
> > > > +	/*
> > > > +	 * Updating of 'bfqd->queued' is protected by 'bfqd->lock', however, it
> > > > +	 * may be read without holding the lock in bfq_has_work().
> > > > +	 */
> > > > +	WRITE_ONCE(bfqd->queued, bfqd->queued + 1);
> > > >
> > > > 	if (RB_EMPTY_ROOT(&bfqq->sort_list) && bfq_bfqq_sync(bfqq)) {
> > > > 		bfq_check_waker(bfqd, bfqq, now_ns);
> > > > @@ -2402,7 +2406,11 @@ static void bfq_remove_request(struct request_queue *q,
> > > > 	if (rq->queuelist.prev != &rq->queuelist)
> > > > 		list_del_init(&rq->queuelist);
> > > > 	bfqq->queued[sync]--;
> > > > -	bfqd->queued--;
> > > > +	/*
> > > > +	 * Updating of 'bfqd->queued' is protected by 'bfqd->lock', however, it
> > > > +	 * may be read without holding the lock in bfq_has_work().
> > > > +	 */
> > > > +	WRITE_ONCE(bfqd->queued, bfqd->queued - 1);
> > > > 	elv_rb_del(&bfqq->sort_list, rq);
> > > >
> > > > 	elv_rqhash_del(q, rq);
> > > > @@ -5063,11 +5071,11 @@ 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
>
> .