linux-kernel - Re: [PATCH-block v2 3/3] blk-cgroup: Flush stats at blkgs destruction path

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Date:   Mon, 12 Dec 2022 19:19:05 -0500
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     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,
        Michal Koutný <mkoutny@...e.com>,
        "Dennis Zhou (Facebook)" <dennisszhou@...il.com>
Subject: Re: [PATCH-block v2 3/3] blk-cgroup: Flush stats at blkgs destruction
 path


On 12/12/22 17:24, Tejun Heo wrote:
> On Sun, Dec 11, 2022 at 05:20:58PM -0500, 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>
>
> But a nit below
>
>> +	/*
>> +	 * Flush all the non-empty percpu lockless lists.
>> +	 */
> Can you please explain the deadlock that's being avoided in the above
> comment? ie. it should say why this flush is necessary.

Sure. I will expand the comment to elaborate a bit more.

Cheers,
Longman

>
>> +	for_each_possible_cpu(cpu) {
>> +		struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu);
>> +
>> +		if (!llist_empty(lhead))
>> +			cgroup_rstat_css_cpu_flush(&blkcg->css, cpu);
>> +	}