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
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