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

[Michal Koutný]

Hello.

On Sun, Dec 11, 2022 at 05:20:57PM -0500, Waiman Long <longman@xxxxxxxxxx> 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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