From 841539281bed5187d2f773097eefb0bb3c5057ec Mon Sep 17 00:00:00 2001
From: Lennart Poettering <lennart@poettering.net>
Date: Tue, 19 Apr 2022 14:03:12 +0800
Subject: [PATCH] cgroup: update only siblings that got realized once

---
 src/core/cgroup.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/src/core/cgroup.c b/src/core/cgroup.c
index f02cc31..e0e0a98 100644
--- a/src/core/cgroup.c
+++ b/src/core/cgroup.c
@@ -1980,7 +1980,16 @@ static void unit_add_siblings_to_cgroup_realize_queue(Unit *u) {
         Unit *slice;
 
         /* This adds the siblings of the specified unit and the siblings of all parent units to the cgroup
-         * queue. (But neither the specified unit itself nor the parents.) */
+	 * queue. (But neither the specified unit itself nor the parents.)
+         *
+         * Propagation of realization "side-ways" (i.e. towards siblings) is in relevant on cgroup-v1 where
+         * scheduling become very weird if two units that own processes reside in the same slice, but one is
+         * realized in the "cpu" hierarchy and once is not (for example because one has CPUWeight= set and
+         * the other does not), because that means processes need to be scheduled against groups. Let's avoid
+         * this asymmetry by always ensuring that units below a slice that are realized at all are hence
+         * always realized in *all* their hierarchies, and it is sufficient for a unit's sibling to be
+         * realized for a unit to be realized too. */
+
 
         while ((slice = UNIT_DEREF(u->slice))) {
                 Iterator i;
@@ -1996,6 +2005,11 @@ static void unit_add_siblings_to_cgroup_realize_queue(Unit *u) {
                         if (UNIT_IS_INACTIVE_OR_FAILED(unit_active_state(m)))
                                 continue;
 
+			/* We only enqueue siblings if they were realized once at least, in the main
+                         * hierarchy. */
+                        if (!m->cgroup_realized)
+                                continue;
+
                         /* If the unit doesn't need any new controllers and has current ones realized, it
                          * doesn't need any changes. */
                         if (unit_has_mask_realized(m,
-- 
2.27.0