linux-kernel - Re: [PATCH v2 2/2] blk-cgroup: Optimize blkcg_rstat

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Message-ID: <6734ae25-27eb-0024-8524-ab8885a5fae0@redhat.com>
Date:   Wed, 1 Jun 2022 17:32:11 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>
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>
Subject: Re: [PATCH v2 2/2] blk-cgroup: Optimize blkcg_rstat_flush()


On 6/1/22 17:28, Tejun Heo wrote:
> On Wed, Jun 01, 2022 at 05:25:53PM -0400, Waiman Long wrote:
>> I think the best way to protect against blkg destruction is to get a percpu
>> reference when put into lockless list and put it back when removed.
>>
>> BTW, when I ran a test that continuously create and destroy containers, the
>> total number of blkcg's kept on increasing. There are some freeing of
>> blkcg's but no freeing of blkg's at all. Maybe we have a similar dying
>> blkcg's problem here. I will take a further look at that when I have time.
> They get pinned by per-cgroup writebacks which gets pinned by lingering page
> cache and other remaining accounted memory areas, so I think they can hang
> around if there's no memory pressure. But, yeah, it'd be great to verify
> that they actually go away under memory pressure.
>
> Thanks.
>
Thanks for the explanation. It makes sense to me.

Cheers,
Longman