Messages in this thread | | | Subject | Re: [PATCH -next v2 2/2] block, bfq: make bfq_has_work() more accurate | From | "yukuai (C)" <> | Date | Wed, 18 May 2022 09:17:23 +0800 |
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在 2022/05/17 23:06, Paolo Valente 写道: > > >> Il giorno 17 mag 2022, alle ore 16:21, Paolo Valente <paolo.valente@linaro.org> ha scritto: >> >> >> >>> Il giorno 16 mag 2022, alle ore 11:56, Jan Kara <jack@suse.cz> 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@huawei.com> >>> >>> Looks good. Feel free to add: >>> >>> Reviewed-by: Jan Kara <jack@suse.cz> >>> >>> 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@suse.com> >>> SUSE Labs, CR >> > > . >
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