linux-kernel - Re: [PATCH -next 2/4] blk-iocost: don't throttle bio if iocg is offlined

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Date:   Tue, 20 Dec 2022 17:38:05 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Tejun Heo <tj@...nel.org>, Yu Kuai <yukuai1@...weicloud.com>
Cc:     hch@...radead.org, josef@...icpanda.com, axboe@...nel.dk,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com,
        "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH -next 2/4] blk-iocost: don't throttle bio if iocg is
 offlined

Hi,

在 2022/12/20 5:28, Tejun Heo 写道:
> On Sat, Dec 17, 2022 at 11:05:25AM +0800, Yu Kuai wrote:
>> From: Yu Kuai <yukuai3@...wei.com>
>>
>> bio will grab blkg reference, however, blkcg->online_pin is not grabbed,
>> hence cgroup can be removed after thread exit while bio is still in
>> progress. Bypass io in this case since it doesn't make sense to
>> throttle bio while cgroup is removed.
> 
> I don't get it. Why wouldn't that make sense? ISTR some occasions where we
> clear the config to mitigate exits stalling for too long but in general a
> policy being active on a draining cgroup shouldn't be a problem.

The main reason here for patch 2/3 is for patch 4, since bio can still
reach rq_qos after pd_offline_fn is called.

Currently, it's not consistent and seems messy how different policies
implement pd_alloc/free_fn, pd_online/offline_fn, and pd_init_fn. For
iocost, iocg is exited in pd_free_fn, and parent iocg can exits before
child, which will cause many problems.

Patch 2/3 are not necessary if we don't choose to fix such problems by
exiting iocg in ioc_pd_offline() in patch 4.

I'll try to think about how to use refcnting, either from blkg layer or
add refcnting for iocg.

Thanks,
Kuai
> 
> Thanks.
>