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 12:16:35 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Waiman Long <longman@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        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

On Mon, Dec 12, 2022 at 01:59:53PM +0100, Michal Koutný wrote:
> 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].)

Yeah, I'm not sure about this patch either. Doesn't seem necessary.

Thanks.

-- 
tejun