Hi Bart

Thanks for your kindly response.

On 06/19/2018 11:18 PM, Bart Van Assche wrote:

On Tue, 2018-06-19 at 15:00 +0800, Jianchao Wang wrote:

quoted

blk_rq_timeout is needed to limit the max timeout value, otherwise,
a idle hctx cannot be deactivated timely in shared-tag case.

Fixes: 12f5b931 (blk-mq: Remove generation seqeunce)
Signed-off-by: Jianchao Wang <redacted>
---
 block/blk-mq.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 70c65bb..ccebe7b 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c

@@ -868,7 +868,7 @@ static void blk_mq_timeout_work(struct work_struct *work)
 	blk_mq_queue_tag_busy_iter(q, blk_mq_check_expired, &next);
 
 	if (next != 0) {
-		mod_timer(&q->timeout, next);
+		mod_timer(&q->timeout, blk_rq_timeout(round_jiffies_up(next)));
 	} else {
 		/*
 		 * Request timeouts are handled as a forward rolling timer. If

Hello Jianchao,

What makes you think that it would be necessary to call blk_rq_timeout() from
blk_mq_timeout_work()? Have you noticed that blk_add_timer() already calls that
function? I think it is not necessary to call blk_rq_timeout() from
blk_mq_timeout_work() because it is guaranteed in that function that the next
timeout is less than BLK_MAX_TIMEOUT jiffies in the future.

blk_add_timer will not re-arm the timer if the timer's expire value is before the new rq's expire value.

Let's look at the following scenario.

0                    +30s
|__________________|___|
T0                T1   T2

T1 = T2 - 1 jiffies 

T0:  rq_a is issued and q->timer is armed and will expire at T2
     then rq_a is completed.
T1:  rq_b is issued and q->timer is not re-armed, because its next expire time is T2 < (T1 + 30s)

T2:  if rq_b have not been completed when timer expires at T2, timer would be re-armed based on the rq_b 
     If we don't have blk_rq_timerout here, the next expire time is about T2 + 30s.
 
This is not good for sharing-tag case.

Thanks
Jianchao