cgroup/rstat: Record the cumulative per-cpu time of cgroup and its descendants

The member variable bstat of the structure cgroup_rstat_cpu
records the per-cpu time of the cgroup itself, but does not
include the per-cpu time of its descendants. The per-cpu time
including descendants is very useful for calculating the
per-cpu usage of cgroups.

Although we can indirectly obtain the total per-cpu time
of the cgroup and its descendants by accumulating the per-cpu
bstat of each descendant of the cgroup. But after a child cgroup
is removed, we will lose its bstat information. This will cause
the cumulative value to be non-monotonic, thus affecting
the accuracy of cgroup per-cpu usage.

So we add the subtree_bstat variable to record the total
per-cpu time of this cgroup and its descendants, which is
similar to "cpuacct.usage*" in cgroup v1. And this is
also helpful for the migration from cgroup v1 to cgroup v2.
After adding this variable, we can obtain the per-cpu time of
cgroup and its descendants in user mode through eBPF/drgn, etc.
And we are still trying to determine how to expose it in the
cgroupfs interface.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Hao Jia <jiahao.os@bytedance.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 14 insertions, 0 deletions

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 8a0d5466c7be..7a2862172f51 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -342,6 +342,20 @@ struct cgroup_rstat_cpu {
 	struct cgroup_base_stat last_bstat;
 
 	/*
+	 * This field is used to record the cumulative per-cpu time of
+	 * the cgroup and its descendants. Currently it can be read via
+	 * eBPF/drgn etc, and we are still trying to determine how to
+	 * expose it in the cgroupfs interface.
+	 */
+	struct cgroup_base_stat subtree_bstat;
+
+	/*
+	 * Snapshots at the last reading. These are used to calculate the
+	 * deltas to propagate to the per-cpu subtree_bstat.
+	 */
+	struct cgroup_base_stat last_subtree_bstat;
+
+	/*
 	 * Child cgroups with stat updates on this cpu since the last read
 	 * are linked on the parent's ->updated_children through
 	 * ->updated_next.