Messages in this thread | | | Subject | Re: [PATCH v2 1/2] blk-iocost: add refcounting for iocg | From | Yu Kuai <> | Date | Mon, 9 Jan 2023 09:32:46 +0800 |
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Hi,
在 2023/01/07 4:18, Tejun Heo 写道: > 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") >
These two commits are applied for three years, I don't check the details yet but they seem can't guarantee that no io will be handled by rq_qos_throttle() after pd_offline_fn(), because I just reproduced this in another problem:
f02be9002c48 ("block, bfq: fix null pointer dereference in bfq_bio_bfqg()")
User thread can issue async io, and io can be throttled by blk-throttle(not writeback), then user thread can exit and cgroup can be removed before such io is dispatched to rq_qos_throttle.
> 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?
Currently I'm sure such IOs can come from blk-throttle, and I'm not sure yet but I also suspect io_uring can do this.
Thanks, Kuai
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