linux-kernel - Re: [PATCH-block v2 2/3] blk-cgroup: Don't flush a blkg if destroyed

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Date:   Mon, 12 Dec 2022 13:59:53 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>,
        Josef Bacik <josef@...icpanda.com>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        "Dennis Zhou (Facebook)" <dennisszhou@...il.com>
Subject: Re: [PATCH-block v2 2/3] blk-cgroup: Don't flush a blkg if destroyed

Hello.

On Sun, Dec 11, 2022 at 05:20:57PM -0500, Waiman Long <longman@...hat.com> wrote:
> Before commit 3b8cc6298724 ("blk-cgroup: Optimize blkcg_rstat_flush()"),
> blkg's stats is only flushed if they are online.

I'm not sure I follow -- css_release_work_fn/cgroup_rstat_flush may be
called on an offlined blkcg (offlined!=released). There's no invariant
ensuring offlined blkcg won't be flushed. (There is only current
situation when there is no reader of io data that'd need them flushed
[1].)

> In addition, the stat flushing of blkgs in blkcg_rstat_flush()
> includes propagating the rstat data to its parent. However, if a blkg
> has been destroyed (offline), the validity of its parent may be
> questionable.

Parents won't be freed (neither offlined) before children (see
css_killed_work_fn). It should be regularly OK to pass data into a
parent of an offlined blkcg.

> For safety, revert back to the old behavior by ignoring offline
> blkg's.

I don't know if this is a good reasoning. If you argue that offlined
children needn't be taken into parent's account, then I think it's more
efficient to exclude the offlined blkcgs from update. (With the caveat I
have in [1].)

Regards,
Michal

[1] https://lore.kernel.org/r/YqEfNgUc8jxlAq8D@blackbook/

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