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

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