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 <[email protected]>
Signed-off-by: Hao Jia <[email protected]>
---
include/linux/cgroup-defs.h | 14 ++++++++++++++
kernel/cgroup/rstat.c | 12 ++++++++++--
2 files changed, 24 insertions(+), 2 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
@@ -341,6 +341,20 @@ 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
diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index 2542c21b6b6d..d80d7a608141 100644
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -344,6 +344,7 @@ static void cgroup_base_stat_flush(struct cgroup *cgrp, int cpu)
{
struct cgroup_rstat_cpu *rstatc = cgroup_rstat_cpu(cgrp, cpu);
struct cgroup *parent = cgroup_parent(cgrp);
+ struct cgroup_rstat_cpu *prstatc;
struct cgroup_base_stat delta;
unsigned seq;
@@ -357,17 +358,24 @@ static void cgroup_base_stat_flush(struct cgroup *cgrp, int cpu)
delta = rstatc->bstat;
} while (__u64_stats_fetch_retry(&rstatc->bsync, seq));
- /* propagate percpu delta to global */
+ /* propagate per-cpu delta to cgroup and per-cpu global statistics */
cgroup_base_stat_sub(&delta, &rstatc->last_bstat);
cgroup_base_stat_add(&cgrp->bstat, &delta);
cgroup_base_stat_add(&rstatc->last_bstat, &delta);
+ cgroup_base_stat_add(&rstatc->subtree_bstat, &delta);
- /* propagate global delta to parent (unless that's root) */
+ /* propagate cgroup and per-cpu global delta to parent (unless that's root) */
if (cgroup_parent(parent)) {
delta = cgrp->bstat;
cgroup_base_stat_sub(&delta, &cgrp->last_bstat);
cgroup_base_stat_add(&parent->bstat, &delta);
cgroup_base_stat_add(&cgrp->last_bstat, &delta);
+
+ delta = rstatc->subtree_bstat;
+ prstatc = cgroup_rstat_cpu(parent, cpu);
+ cgroup_base_stat_sub(&delta, &rstatc->last_subtree_bstat);
+ cgroup_base_stat_add(&prstatc->subtree_bstat, &delta);
+ cgroup_base_stat_add(&rstatc->last_subtree_bstat, &delta);
}
}
--
2.37.0
On Mon, Aug 07, 2023 at 11:29:30AM +0800, Hao Jia wrote:
> 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 <[email protected]>
> Signed-off-by: Hao Jia <[email protected]>
Applied to cgroup/for-6.6.
Thanks.
--
tejun