linux-kernel - Re: [External] Re: [PATCH v2] blk-throttle: Fix io statistics for cgroup v1

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Date:   Fri, 5 May 2023 21:35:21 +0800
From:   hanjinke <hanjinke.666@...edance.com>
To:     Andrea Righi <andrea.righi@...onical.com>
Cc:     tj@...nel.org, josef@...icpanda.com, axboe@...nel.dk,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [External] Re: [PATCH v2] blk-throttle: Fix io statistics for
 cgroup v1



在 2023/5/5 上午5:13, Andrea Righi 写道:
> On Thu, May 04, 2023 at 11:08:53PM +0800, hanjinke wrote:
>> Hi
>>
>> Sorry for delay（Chinese Labor Day holiday).
> 
> No problem, it was also Labor Day in Italy. :)
> 
>>
>> 在 2023/4/29 上午3:05, Andrea Righi 写道:
>>> On Sat, Apr 01, 2023 at 05:47:08PM +0800, Jinke Han wrote:
>>>> From: Jinke Han <hanjinke.666@...edance.com>
>>>>
>>>> After commit f382fb0bcef4 ("block: remove legacy IO schedulers"),
>>>> blkio.throttle.io_serviced and blkio.throttle.io_service_bytes become
>>>> the only stable io stats interface of cgroup v1, and these statistics
>>>> are done in the blk-throttle code. But the current code only counts the
>>>> bios that are actually throttled. When the user does not add the throttle
>>>> limit, the io stats for cgroup v1 has nothing. I fix it according to the
>>>> statistical method of v2, and made it count all ios accurately.
>>>>
>>>> Fixes: a7b36ee6ba29 ("block: move blk-throtl fast path inline")
>>>> Signed-off-by: Jinke Han <hanjinke.666@...edance.com>
>>>
>>> Thanks for fixing this!
>>>
>>> The code looks correct to me, but this seems to report io statistics
>>> only if at least one throttling limit is defined. IIRC with cgroup v1 it
>>> was possible to see the io statistics inside a cgroup also with no
>>> throttling limits configured.
>>>
>>> Basically to restore the old behavior we would need to drop the
>>> cgroup_subsys_on_dfl() check, something like the following (on top of
>>> your patch).
>>>
>>> But I'm not sure if we're breaking other behaviors in this way...
>>> opinions?
>>>
>>>    block/blk-cgroup.c   |  3 ---
>>>    block/blk-throttle.h | 12 +++++-------
>>>    2 files changed, 5 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
>>> index 79138bfc6001..43af86db7cf3 100644
>>> --- a/block/blk-cgroup.c
>>> +++ b/block/blk-cgroup.c
>>> @@ -2045,9 +2045,6 @@ void blk_cgroup_bio_start(struct bio *bio)
>>>    	struct blkg_iostat_set *bis;
>>>    	unsigned long flags;
>>> -	if (!cgroup_subsys_on_dfl(io_cgrp_subsys))
>>> -		return;
>>> -
>>>    	/* Root-level stats are sourced from system-wide IO stats */
>>>    	if (!cgroup_parent(blkcg->css.cgroup))
>>>    		return;
>>> diff --git a/block/blk-throttle.h b/block/blk-throttle.h
>>> index d1ccbfe9f797..bcb40ee2eeba 100644
>>> --- a/block/blk-throttle.h
>>> +++ b/block/blk-throttle.h
>>> @@ -185,14 +185,12 @@ static inline bool blk_should_throtl(struct bio *bio)
>>>    	struct throtl_grp *tg = blkg_to_tg(bio->bi_blkg);
>>>    	int rw = bio_data_dir(bio);
>>> -	if (!cgroup_subsys_on_dfl(io_cgrp_subsys)) {
>>> -		if (!bio_flagged(bio, BIO_CGROUP_ACCT)) {
>>> -			bio_set_flag(bio, BIO_CGROUP_ACCT);
>>> -			blkg_rwstat_add(&tg->stat_bytes, bio->bi_opf,
>>> -					bio->bi_iter.bi_size);
>>> -		}
>>> -		blkg_rwstat_add(&tg->stat_ios, bio->bi_opf, 1);
>>> +	if (!bio_flagged(bio, BIO_CGROUP_ACCT)) {
>>> +		bio_set_flag(bio, BIO_CGROUP_ACCT);
>>> +		blkg_rwstat_add(&tg->stat_bytes, bio->bi_opf,
>>> +				bio->bi_iter.bi_size);
>>>    	}
>>> +	blkg_rwstat_add(&tg->stat_ios, bio->bi_opf, 1);
>>

I checked the code again. If we remove cgroup_subsys_on_dfl check here, 
io statistics will still be performed in the case of v2, which I think 
is unnecessary, and this information will be counted to 
io_service_bytes/io_serviced, these two files are not visible in v2. Am 
I missing something?

Thanks.
Jinke