Re: [PATCH v2 1/2] blk-iocost: add refcounting for iocg

[Tejun Heo]

On Fri, Jan 06, 2023 at 09:08:45AM +0800, Yu Kuai wrote:
> Hi,
> 
> 在 2023/01/06 2:32, Tejun Heo 写道:
> > On Thu, Jan 05, 2023 at 09:14:07AM +0800, Yu Kuai wrote:
> > > 1) is related to blkg, while 2) is not, hence refcnting from blkg can't
> > > fix the problem. refcnting from blkcg_policy_data should be ok, but I
> > > see that bfq already has the similar refcnting, while other policy
> > > doesn't require such refcnting.
> > 
> > Hmm... taking a step back, wouldn't this be solved by moving the first part
> > of ioc_pd_free() to pd_offline_fn()? The ordering is strictly defined there,
> > right?
> > 
> 
> Moving first part to pd_offline_fn() has some requirements, like what I
> did in the other thread:
> 
> iocg can be activated again after pd_offline_fn(), which is possible
> because bio can be dispatched when cgroup is removed. I tried to avoid
> that by:
> 
> 1) dispatch all throttled bio io ioc_pd_offline()
> 2) don't throttle bio after ioc_pd_offline()
> 
> However, you already disagreed with that. 😔

Okay, I was completely wrong while I was replying to your original patch.
Should have looked at the code closer, my apologies.

What I missed is that pd_offline doesn't happen when the cgroup goes
offline. Please take a look at the following two commits:

 59b57717fff8 ("blkcg: delay blkg destruction until after writeback has finished")
 d866dbf61787 ("blkcg: rename blkcg->cgwb_refcnt to ->online_pin and always use it")

After the above two commits, ->pd_offline_fn() is called only after all
possible writebacks are complete, so it shouldn't allow mass escapes to
root. With writebacks out of the picture, it might be that there can be no
further IOs once ->pd_offline_fn() is called too as there can be no tasks
left in it and no dirty pages, but best to confirm that.

So, yeah, the original approach you took should work although I'm not sure
the patches that you added to make offline blkg to bypass are necessary
(that also contributed to my assumption that there will be more IOs on those
blkg's). Have you seen more IOs coming down the pipeline after offline? If
so, can you dump some backtraces and see where they're coming from?

Thanks.

-- 
tejun