linux-kernel - Re: cgroup io.stat propagation regression

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Date: Wed, 15 May 2024 10:26:31 -0400
From: Waiman Long <longman@...hat.com>
To: Dan Schatzberg <schatzberg.dan@...il.com>, Tejun Heo <tj@...nel.org>
Cc: linux-block@...r.kernel.org, cgroups@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: cgroup io.stat propagation regression


On 5/14/24 23:59, Waiman Long wrote:
> On 5/14/24 15:25, Dan Schatzberg wrote:
>> Hi Waiman,
>>
>> I've noticed that on recent kernels io.stat metrics don't propagate
>> all the way up the hierarchy. Specifically, io.stat metrics of some
>> leaf cgroup will be propagated to the parent, but not its grandparent.
>>
>> For a simple repro, run the following:
>>
>> systemd-run --slice test-test dd if=/dev/urandom of=/tmp/test bs=4096 
>> count=1
>>
>> Then:
>>
>> cat /sys/fs/cgroup/test.slice/test-test.slice/io.stat
>>
>> Shows the parent cgroup stats and I see wbytes=4096 but the 
>> grandparent cgroup:
>>
>> cat /sys/fs/cgroup/test.slice/io.stat
>>
>> shows no writes.
>>
>> I believe this was caused by the change in "blk-cgroup: Optimize
>> blkcg_rstat_flush()". When blkcg_rstat_flush is called on the parent
>> cgroup, it exits early because the lockless list is empty since the
>> parent cgroup never issued writes itself (e.g. in
>> blk_cgroup_bio_start). However, in doing so it never propagated stats
>> to its parent.
>>
>> Can you confirm if my understanding of the logic here is correct and
>> advise on a fix?
>
> Yes, I believe your analysis is correct. Thanks for spotting this 
> iostat propagation problem.
>
> I am working on a fix to address this problem and will post a patch 
> once I have finished my testing.

Actually, I can only reproduce the issue with a 3-level 
(child-parent-grandparent) cgroup hierarchy below the root cgroup. The 
dd command is run test.slice/test-test.slice. So both test.slice/io.stat 
and test.slice/test-test.slice/io.stat are properly updated.

Cheers,
Longman