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

[Paolo Valente <paolo.valente@linaro.org>]

> 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 >=).  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