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

[Tejun Heo]

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@xxxxxxxxxx

> 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.

>  		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?

Thanks.

-- 
tejun