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

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Message-ID: <ZkToxDuKxSPEg5aP@dschatzberg-fedora-PF3DHTBV>
Date: Wed, 15 May 2024 12:54:28 -0400
From: Dan Schatzberg <schatzberg.dan@...il.com>
To: Waiman Long <longman@...hat.com>
Cc: Tejun Heo <tj@...nel.org>, linux-block@...r.kernel.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: cgroup io.stat propagation regression

On Wed, May 15, 2024 at 10:26:31AM -0400, Waiman Long wrote:
> 
> 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.

That's correct, this repros with a 3-level cgroup hierarchy (or
more). systemd-run should create an ephemeral .scope cgroup under
test-test.slice and then delete it when the dd command finishes. So
test.slice/test-test.slice was the parent (2nd level) and test.slice
is the grandparent.