Re: [PATCH] blk-mq: use blk_mq_timeout_work to limit the max timeout

["jianchao.wang" <jianchao.w.wang@oracle.com>]

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:
>> 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 <jianchao.w.wang@oracle.com>
>> ---
>>  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

> 
>