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

[Yu Kuai]

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@xxxxxxxxxx>
> >
> > 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.