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

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

On 06/20/2018 09:35 AM, Bart Van Assche wrote:
> On Wed, 2018-06-20 at 09:28 +0800, jianchao.wang wrote:
>> 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_timeout here, the next expire time is about T2 + 30s.
> 
> Hello Jianchao,
> 
> I disagree with the last sentence above. I think for your example blk_mq_req_expired()
> will set next to T1 + 30s instead of T2 + 30s.
> 

Would you please explain the reason ?

Thanks
Jianchao

> Bart.
> 
>