linux-kernel - Re: [PATCH v10 3/3] blk-cgroup: Flush stats at blkgs destruction path

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Date:   Mon, 14 Nov 2022 12:43:57 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ming Lei <ming.lei@...hat.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Koutný <mkoutny@...e.com>,
        Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH v10 3/3] blk-cgroup: Flush stats at blkgs destruction path

On Fri, Nov 04, 2022 at 08:59:02PM -0400, Waiman Long wrote:
> As noted by Michal, the blkg_iostat_set's in the lockless list
> hold reference to blkg's to protect against their removal. Those
> blkg's hold reference to blkcg. When a cgroup is being destroyed,
> cgroup_rstat_flush() is only called at css_release_work_fn() which is
> called when the blkcg reference count reaches 0. This circular dependency
> will prevent blkcg from being freed until some other events cause
> cgroup_rstat_flush() to be called to flush out the pending blkcg stats.
> 
> To prevent this delayed blkcg removal, add a new cgroup_rstat_css_flush()
> function to flush stats for a given css and cpu and call it at the blkgs
> destruction path, blkcg_destroy_blkgs(), whenever there are still some
> pending stats to be flushed. This will ensure that blkcg reference
> count can reach 0 ASAP.
> 
> Signed-off-by: Waiman Long <longman@...hat.com>

Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun