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.
Thanks, Longman
