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

[Gui Jianfeng <guijianfeng@cn.fujitsu.com>]

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.

#!/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@cn.fujitsu.com>
>> ---
>>  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