> 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.
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.
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
>
