[PATCH v2 -next 3/3] cgroup/freezer: Reduce redundant propagation for cgroup_propagate_frozen

[Chen Ridong <chenridong@xxxxxxxxxx>]

When a cgroup is frozen/unfrozen, it will always propagate bottom up.
However it is unnecessary to propagate to the top every time. This patch
aims to reduce redundant propagation for cgroup_propagate_frozen.

For example, subtree like:
	a
	|
	b
      / | \
     c  d  e
If c is frozen, and d and e are not frozen now, it doesn't have to
propagate to a; Only when c, d and e are all frozen, b and a could be set
to frozen. Therefore, if nr_frozen_descendants is not equal to
nr_descendants, just stop propagate. If a descendant is frozen, the
parent's nr_frozen_descendants add child->nr_descendants + 1. This can
reduce redundant propagation.

Additionally, cgroup_propagate_frozen is not only used to update the
ancestor state but also to update itself. This approach can make the code
clearer and significantly simplify cgroup_update_frozen.

Signed-off-by: Chen Ridong <chenridong@xxxxxxxxxx>
---
 include/linux/cgroup-defs.h |  4 +++-
 kernel/cgroup/freezer.c     | 29 +++++++++++++++++------------
 2 files changed, 20 insertions(+), 13 deletions(-)

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index dd1ecab99eeb..41e4e5a7ae55 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -401,7 +401,9 @@ struct cgroup_freezer_state {
 
 	/* Fields below are protected by css_set_lock */
 
-	/* Number of frozen descendant cgroups */
+	/* Aggregating frozen descendant cgroups, only when all
+	 * descendants of a child are frozen will the count increase.
+	 */
 	int nr_frozen_descendants;
 
 	/*
diff --git a/kernel/cgroup/freezer.c b/kernel/cgroup/freezer.c
index bf1690a167dd..0b9d20012a5a 100644
--- a/kernel/cgroup/freezer.c
+++ b/kernel/cgroup/freezer.c
@@ -35,27 +35,34 @@ static bool cgroup_update_frozen_flag(struct cgroup *cgrp, bool frozen)
  */
 static void cgroup_propagate_frozen(struct cgroup *cgrp, bool frozen)
 {
-	int desc = 1;
-
+	int deta;
+	struct cgroup *parent;
 	/*
 	 * If the new state is frozen, some freezing ancestor cgroups may change
 	 * their state too, depending on if all their descendants are frozen.
 	 *
 	 * Otherwise, all ancestor cgroups are forced into the non-frozen state.
 	 */
-	while ((cgrp = cgroup_parent(cgrp))) {
+	for (cgrp = cgrp; cgrp; cgrp = cgroup_parent(cgrp)) {
 		if (frozen) {
-			cgrp->freezer.nr_frozen_descendants += desc;
+			/* If freezer is not set, or cgrp has descendants
+			 * that are not frozen, cgrp can't be frozen
+			 */
 			if (!test_bit(CGRP_FREEZE, &cgrp->flags) ||
 			    (cgrp->freezer.nr_frozen_descendants !=
-			    cgrp->nr_descendants))
-				continue;
+			     cgrp->nr_descendants))
+				break;
+			deta = cgrp->freezer.nr_frozen_descendants + 1;
 		} else {
-			cgrp->freezer.nr_frozen_descendants -= desc;
+			deta = -(cgrp->freezer.nr_frozen_descendants + 1);
 		}
 
-		if (cgroup_update_frozen_flag(cgrp, frozen))
-			desc++;
+		/* No change, stop propagate */
+		if (!cgroup_update_frozen_flag(cgrp, frozen))
+			break;
+
+		parent = cgroup_parent(cgrp);
+		parent->freezer.nr_frozen_descendants += deta;
 	}
 }
 
@@ -75,9 +82,7 @@ void cgroup_update_frozen(struct cgroup *cgrp)
 	frozen = test_bit(CGRP_FREEZE, &cgrp->flags) &&
 		cgrp->freezer.nr_frozen_tasks == __cgroup_task_count(cgrp);
 
-	/* If flags is updated, update the state of ancestor cgroups. */
-	if (cgroup_update_frozen_flag(cgrp, frozen))
-		cgroup_propagate_frozen(cgrp, frozen);
+	cgroup_propagate_frozen(cgrp, frozen);
 }
 
 /*
-- 
2.34.1