Re:Re: cgroup: Fix split bio been throttled more than once

[aaronlee0817 <aaronlee0817@163.com>]

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Hi Tejun Heo

>At 2016-07-06 06:26:26, "Tejun Heo" <tj@kernel.org> wrote:
>Hello,
>
>On Mon, Jul 04, 2016 at 10:46:54PM +0800, Jiale Li wrote:
>> These days, we have tested the cgroup blkio throttle function use fio,
>> and we found a problem that when we use buffered IO or set the big block
>> size like 1M, then the IO performance cannot reach the value we set.
>> For example we set blkio.throttle.read_bps_device as 10M, in kernel 
>> version 4.3 IO performance can only reach 6M, and in kernel version 
>> 4.4 the actual IO bps is only 3.1M. 
>> 
>> Then we did some research and find that in kernel version 4.3 brought in
>> blk_queue_split() function to split the big size bio into several parts,
>> and some of them are calling the generic_make_request() again, this result
>> the bio been throttled more than once. so the actual bio sent to device is 
>> less than we expected.
>> 
>> We have checked the newest kernel of 4.7-rc5, this problem is still exist.
>> 
>> Based on this kind of situation, we propose a fix solution to add a flag bit
>> in bio to let the splited bio bypass the blk_queue_split(). Below is the patch
>> we used to fix this problem.
>
>Thanks a lot for the report.  Hmmm... there was another brekage around
>split bios, I wonder whether the two can be handled together somehow.
>
> http://lkml.kernel.org/g/1466583730-28595-1-git-send-email-lars.ellenberg@linbit.com 


About this patch, I agree with Ming Lei, this is another issue, they are independent
of each other.
  
>
>> diff --git a/block/blk-merge.c b/block/blk-merge.c
>> index 2613531..7b17a65 100644
>> --- a/block/blk-merge.c
>> +++ b/block/blk-merge.c
>> @@ -190,6 +190,7 @@ void blk_queue_split(struct request_queue *q, struct bio **bio,
>>  
>>  bio_chain(split, *bio);
>>  trace_block_split(q, split, (*bio)->bi_iter.bi_sector);
>> +bio_set_flag(*bio, BIO_SPLITED);
>
>Split's past participle form is split, so BIO_SPLIT would be right
>here.


Yea, you are right, it should be BIO_SPLIT


>
>>  generic_make_request(*bio);
>>  *bio = split;
>>  }
>> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
>> index 47a3e54..4ffde95 100644
>> --- a/block/blk-throttle.c
>> +++ b/block/blk-throttle.c
>> @@ -1403,6 +1403,10 @@ bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg,
>>  
>>  WARN_ON_ONCE(!rcu_read_lock_held());
>>  
>> +/* if the bio has been splited, should not throttle again */
>> +if (bio_flagged(bio, BIO_SPLITED))
>> +goto out;
>> +
>
>But that said, can't we just copy BIO_THROTLED while splitting?


In my opinion， copying BIO_THROTLED while splitting doesn't work
with this, because, the bio within the limit will not set BIO_THROTLED
but will be split, and back to call generic_make_request(), and will be 
count twice.


Thanks,


Jiale


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