Re: [PATCH 02/20] blkio: Change CFQ to use CFS like queue time stamps

[Gui Jianfeng]

Vivek Goyal wrote:
> On Fri, Nov 13, 2009 at 08:59:08AM +0800, Gui Jianfeng wrote:
>>
>> Vivek Goyal wrote:
>>> On Wed, Nov 11, 2009 at 08:48:09AM +0800, Gui Jianfeng wrote:
>>>> Vivek Goyal wrote:
>>>> ...
>>>>>  
>>>>> @@ -1245,10 +1429,10 @@ static int cfq_forced_dispatch(struct cfq_data *cfqd)
>>>>>  	struct cfq_queue *cfqq;
>>>>>  	int dispatched = 0;
>>>>>  
>>>>> -	while ((cfqq = cfq_rb_first(&cfqd->service_tree)) != NULL)
>>>>> +	while ((cfqq = cfq_get_next_queue(cfqd)) != NULL)
>>>>>  		dispatched += __cfq_forced_dispatch_cfqq(cfqq);
>>>>>  
>>>>> -	cfq_slice_expired(cfqd, 0);
>>>>> +	cfq_slice_expired(cfqd);
>>>>>  
>>>>>  	BUG_ON(cfqd->busy_queues);
>>>>>  
>>>>> @@ -1391,7 +1575,7 @@ static int cfq_dispatch_requests(struct request_queue *q, int force)
>>>>>  	    cfqq->slice_dispatch >= cfq_prio_to_maxrq(cfqd, cfqq)) ||
>>>>>  	    cfq_class_idle(cfqq))) {
>>>>>  		cfqq->slice_end = jiffies + 1;
>>>>> -		cfq_slice_expired(cfqd, 0);
>>>>> +		cfq_slice_expired(cfqd);
>>>> Hi Vivek,
>>>>
>>>> I think here you should make sure that when updating cfqq->slice_end, cfqq->slice_end doesn't 
>>>> equal to 0. Because if cfqq->slice_end == 0, cfq_slice_expired() just charge for 1 jiffy, but
>>>> if cfqq->slice_end is updated when it equals to 0(first request still in the air), at that time
>>>> cfqq->slice_start == 0, and slice_used is charged as "jiffies - cfqq->slice_start". Following
>>>> patch fixes this bug.
>>>>
>>> Hi Gui,
>>>
>>> This can happen only once during a one wrap around cycle of jiffies. That
>>> too depends in case we are hitting jiffies+1 as 0 or not.
>>>
>>> So I would not worry much about it right now.
>>>
>>> In fact, not updating slice_end, will make idle or async queue slice last
>>> much longer than it should have.
>>   I don't think so Vivek, this bug can be easily trigger by creating two cgroup and run a idle
>>   task in one group, then run a normal task in the other group. When the idle task sends out its
>>   first request, this bug occurs. I can reproduce this bug every time by the following script.
>>
> 
> Oh.., sorry, Looks like I read your mail too fast. So you are saying that
> in this case we should be charging 1 ms but instead we will be charging
> (jiffies - 0), which might be too huge a number and then a particular
> group will not be scheduled for a long time?

  Yes, that's it.

> 
> How about changing the charging code to also check if slice_start == 0? So
> in my V2 I will change the cfq_cfqq_slice_usage() to also check for
> slice_start to make sure whether a slice has actually started or not.
> 
> if (!cfqq->slice_start || cfqq->slice_start == jiffies) {
> 	charge_1ms;
> else
> 	charge_based_on_time_elapsed;

  I think this change should also work :)

Thanks
Gui

> 
> Thanks
> Vivek
> 
>> #!/bin/sh
>>
>> mkdir /cgroup
>> mount -t cgroup -o blkio io /cgroup
>> mkdir /cgroup/tst1
>> mkdir /cgroup/tst2
>>
>> dd if=/dev/sdb2 of=/dev/null &
>> pid1=$!
>> echo $pid1 > /cgroup/tst1/tasks
>>
>> dd if=/dev/sdb3 of=/dev/null &
>> pid2=$!
>> ionice -c3 -p$pid2
>> echo $pid2 > /cgroup/tst2/tasks
>>
>> sleep 5
>>
>> cat /cgroup/tst1/blkio.time
>> cat /cgroup/tst2/blkio.time
>>
>> killall -9 dd
>> sleep 1
>>
>> rmdir /cgroup/tst1
>> rmdir /cgroup/tst2
>> umount /cgroup
>> rmdir /cgroup
>>
>>
>>> Thanks
>>> Vivek
>>>
>>>  
>>>> Signed-off-by: Gui Jianfeng <guijianfeng@xxxxxxxxxxxxxx>
>>>> ---
>>>>  block/cfq-iosched.c |    3 ++-
>>>>  1 files changed, 2 insertions(+), 1 deletions(-)
>>>>
>>>> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
>>>> index f23d713..12afc14 100644
>>>> --- a/block/cfq-iosched.c
>>>> +++ b/block/cfq-iosched.c
>>>> @@ -1999,7 +1999,8 @@ static int cfq_dispatch_requests(struct request_queue *q, int force)
>>>>  	if (cfqd->busy_queues > 1 && ((!cfq_cfqq_sync(cfqq) &&
>>>>  	    cfqq->slice_dispatch >= cfq_prio_to_maxrq(cfqd, cfqq)) ||
>>>>  	    cfq_class_idle(cfqq))) {
>>>> -		cfqq->slice_end = jiffies + 1;
>>>> +		if (cfqq->slice_end)
>>>> +			cfqq->slice_end = jiffies + 1;
>>>>  		cfq_slice_expired(cfqd);
>>>>  	}
>>>>  
>>>> -- 
>>>> 1.5.4.rc3
>>>
>>>
>> -- 
>> Regards
>> Gui Jianfeng
> 
> 
> 

-- 
Regards
Gui Jianfeng

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