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Subject: Re: [PATCH v2 1/2] blk-iocost: add refcounting for iocg
To:     Tejun Heo <tj@kernel.org>, Yu Kuai <yukuai1@huaweicloud.com>
Cc:     hch@infradead.org, josef@toxicpanda.com, axboe@kernel.dk,
        cgroups@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, yi.zhang@huawei.com,
        "yukuai (C)" <yukuai3@huawei.com>
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From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <efa1c73b-e94f-373f-e535-2cfc32ce2433@huaweicloud.com>
Date:   Fri, 13 Jan 2023 09:10:25 +0800
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Hi,

?? 2023/01/13 8:53, Tejun Heo д??:
> Hello,
> 
> On Thu, Jan 12, 2023 at 02:18:15PM +0800, Yu Kuai wrote:
>> remove the blkcg_deactivate_policy() from rq_qos_exit() from deleting
>> the device, and delay the policy cleanup and free to blkg_destroy_all().
>> Then the policies(other than bfq) can only call pd_free_fn() from
>> blkg_destroy(), and it's easy to guarantee the order. For bfq, it can
>> stay the same since bfq has refcounting itself.
>>
>> Then for the problem that ioc can be freed in pd_free_fn(), we can fix
>> it by freeing ioc in ioc_pd_free() for root blkg instead of
>> rq_qos_exit().
>>
>> What do you think?
> 
> That would remove the ability to dynamically remove an rq_qos policy, right?
> We don't currently do it but given that having an rq_qos registered comes
> with perf overhead, it's something we might want to do in the future - e.g.

Yes, that make sense, remove ioc and other policies dynamically.

> only activate the policy when the controller is actually enabled. So, idk.
> What's wrong with synchronizing the two removal paths? blkcg policies are
> combinations of cgroups and block device configurations, so having exit
> paths from both sides is kinda natural.

I still can't figure out how to synchronizing them will a mutex. Maybe
I'm being foolish...

Thanks,
Kuai