core: cache last CPU usage counter, before destorying a cgroup

It is useful for clients to be able to read the last CPU usage counter value of
a unit even if the unit is already terminated. Hence, before destroying a
cgroup's cgroup cache the last CPU usage counter and return it if the cgroup is
gone.

3 files changed, 37 insertions, 3 deletions

diff --git a/src/core/cgroup.c b/src/core/cgroup.c
index 086cd7a789..20f9f9fc7e 100644
--- a/src/core/cgroup.c
+++ b/src/core/cgroup.c
@@ -1524,6 +1524,8 @@ void unit_prune_cgroup(Unit *u) {
         if (!u->cgroup_path)
                 return;
 
+        (void) unit_get_cpu_usage(u, NULL); /* Cache the last CPU usage value before we destroy the cgroup */
+
         is_root_slice = unit_has_name(u, SPECIAL_ROOT_SLICE);
 
         r = cg_trim_everywhere(u->manager->cgroup_supported, u->cgroup_path, !is_root_slice);
@@ -2044,7 +2046,21 @@ int unit_get_cpu_usage(Unit *u, nsec_t *ret) {
         nsec_t ns;
         int r;
 
+        assert(u);
+
+        /* Retrieve the current CPU usage counter. This will subtract the CPU counter taken when the unit was
+         * started. If the cgroup has been removed already, returns the last cached value. To cache the value, simply
+         * call this function with a NULL return value. */
+
         r = unit_get_cpu_usage_raw(u, &ns);
+        if (r == -ENODATA && u->cpu_usage_last != NSEC_INFINITY) {
+                /* If we can't get the CPU usage anymore (because the cgroup was already removed, for example), use our
+                 * cached value. */
+
+                if (ret)
+                        *ret = u->cpu_usage_last;
+                return 0;
+        }
         if (r < 0)
                 return r;
 
@@ -2053,7 +2069,10 @@ int unit_get_cpu_usage(Unit *u, nsec_t *ret) {
         else
                 ns = 0;
 
-        *ret = ns;
+        u->cpu_usage_last = ns;
+        if (ret)
+                *ret = ns;
+
         return 0;
 }
 
@@ -2063,6 +2082,8 @@ int unit_reset_cpu_usage(Unit *u) {
 
         assert(u);
 
+        u->cpu_usage_last = NSEC_INFINITY;
+
         r = unit_get_cpu_usage_raw(u, &ns);
         if (r < 0) {
                 u->cpu_usage_base = 0;
diff --git a/src/core/unit.c b/src/core/unit.c
index 6d92eb0c30..03ee6424fa 100644
--- a/src/core/unit.c
+++ b/src/core/unit.c
@@ -102,6 +102,7 @@ Unit *unit_new(Manager *m, size_t size) {
         u->job_timeout = USEC_INFINITY;
         u->ref_uid = UID_INVALID;
         u->ref_gid = GID_INVALID;
+        u->cpu_usage_last = NSEC_INFINITY;
 
         RATELIMIT_INIT(u->start_limit, m->default_start_limit_interval, m->default_start_limit_burst);
         RATELIMIT_INIT(u->auto_stop_ratelimit, 10 * USEC_PER_SEC, 16);
@@ -2620,7 +2621,10 @@ int unit_serialize(Unit *u, FILE *f, FDSet *fds, bool serialize_jobs) {
                 unit_serialize_item(u, f, "assert-result", yes_no(u->assert_result));
 
         unit_serialize_item(u, f, "transient", yes_no(u->transient));
+
         unit_serialize_item_format(u, f, "cpu-usage-base", "%" PRIu64, u->cpu_usage_base);
+        if (u->cpu_usage_last != NSEC_INFINITY)
+                unit_serialize_item_format(u, f, "cpu-usage-last", "%" PRIu64, u->cpu_usage_last);
 
         if (u->cgroup_path)
                 unit_serialize_item(u, f, "cgroup", u->cgroup_path);
@@ -2843,11 +2847,19 @@ int unit_deserialize(Unit *u, FILE *f, FDSet *fds) {
 
                         continue;
 
-                } else if (streq(l, "cpu-usage-base") || streq(l, "cpuacct-usage-base")) {
+                } else if (STR_IN_SET(l, "cpu-usage-base", "cpuacct-usage-base")) {
 
                         r = safe_atou64(v, &u->cpu_usage_base);
                         if (r < 0)
-                                log_unit_debug(u, "Failed to parse CPU usage %s, ignoring.", v);
+                                log_unit_debug(u, "Failed to parse CPU usage base %s, ignoring.", v);
+
+                        continue;
+
+                } else if (streq(l, "cpu-usage-last")) {
+
+                        r = safe_atou64(v, &u->cpu_usage_last);
+                        if (r < 0)
+                                log_unit_debug(u, "Failed to read CPU usage last %s, ignoring.", v);
 
                         continue;
 
diff --git a/src/core/unit.h b/src/core/unit.h
index e5a2a77b7b..31f0fef87b 100644
--- a/src/core/unit.h
+++ b/src/core/unit.h
@@ -194,6 +194,7 @@ struct Unit {
 
         /* Where the cpu.stat or cpuacct.usage was at the time the unit was started */
         nsec_t cpu_usage_base;
+        nsec_t cpu_usage_last; /* the most recently read value */
 
         /* Counterparts in the cgroup filesystem */
         char *cgroup_path;