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

[Ming Lei]

On Wed, Jul 6, 2016 at 10:09 PM, Tejun Heo <tj@xxxxxxxxxx> wrote:
> Hello, Ming.
>
> On Wed, Jul 06, 2016 at 09:10:00AM +0800, Ming Lei wrote:
>> > 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.
>>
>> Except for blk_queue_split(), there are other(stacked) drivers which call
>> generic_make_request() too, such as drbd, dm, md and bcache.
>
> So, blk-throtl already uses REQ_THROTTLED to avoid throttling the same
> bio multiple times.  The problem seems that the flag isn't maintained
> through clone.

Actually the flag(bio->bi_rw) has been maintained during clone, please
see __bio_clone_fast() and bio_clone_bioset().

>
>> >
>> > 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.
>>
>> The splitted bio is just a fast-cloned bio(except for discard bio) and not very
>> special compared with other fast-cloned bio, which is quite common used.
>>
>> So I guess what you need is to bypass BIO_CLONED bio for this purpose
>> since all fast-cloned bio shares the same bvec table of the source bio.
>
> Depending on how a device handles a bio, that could allow bios to
> bypass throttling entirely, no?  Wouldn't adding REQ_THROTTLED to
> REQ_CLONE_MASK work?
>
> Thanks.
>
> --
> tejun


Thanks,
Ming Lei