linux-kernel - Re: [PATCH v2] blk-cgroup: don't clear stat in blkcg_reset

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Message-ID: <eedc01d5-42f9-39f0-25c4-340f35ee8eab@huaweicloud.com>
Date: Tue, 22 Oct 2024 11:44:33 +0800
From: Li Lingfeng <lilingfeng@...weicloud.com>
To: Michal Koutný <mkoutny@...e.com>
Cc: tj@...nel.org, josef@...icpanda.com, hch@....de, axboe@...nel.dk,
 longman@...hat.com, ming.lei@...hat.com, cgroups@...r.kernel.org,
 linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 yukuai1@...weicloud.com, houtao1@...wei.com, yi.zhang@...wei.com,
 yangerkun@...wei.com, yukuai3@...wei.com, lilingfeng3@...wei.com
Subject: Re: [PATCH v2] blk-cgroup: don't clear stat in blkcg_reset_stats()


在 2024/8/21 23:51, Michal Koutný 写道:
> Hello.
>
> On Wed, Aug 21, 2024 at 10:07:56AM GMT, Li Lingfeng <lilingfeng@...weicloud.com> wrote:
>> The list of root cgroup can be used by both cgroup v1 and v2 while
>> non-root cgroup can't since it must be removed before switch between
>> cgroup v1 and v2.
>> So it may has effect if the list of root used by cgroup v2 was corrupted
>> after switching to cgroup v1, and switch back to cgroup v2 to use the
>> corrupted list again.
>> However, the root cgroup will not use the list any more after commit
>> ef45fe470e1e("blk-cgroup: show global disk stats in root cgroup io.stat").
> How come? Before that patch the root file was excluded with
> CFTYPE_NOT_ON_ROOT. IOW how has that patch an effect on llist traversal?
>
> (It doesn't matter, your patch doesn't restore memset anyway.)
>
> This is the reasoning how I understand it:
>
> | The removed function clears blkg.iostat structures that is only used on
> | v2 whereas the function can only be called with v1 hierarchy attached.
> | Zeroing effect could potentially be visible root blkcg "shared" between
> | v1 and v2 but v2 actually synthesizes stats differently for root.
Yes, that's what I mean.
>> Although this has no negative effect, it is not necessary. Remove the
>> related code for cleanup. No function change.
> I'm impressed by the amount of analysis you did to potentially remove
> the unused function. If you feel like cleaning up more or sectioning,
> see also [1] or [2] for inspiration.
Thank you for your advice. I'll look at them later.
> Thanks,
> Michal
>
> [1] https://lore.kernel.org/all/20240625005906.106920-14-roman.gushchin@linux.dev/
> [2] https://lore.kernel.org/all/20240628210317.272856-1-roman.gushchin@linux.dev/