cgroup: implement cgroup_subsys->depends_on

Currently, the blkio subsystem attributes all of writeback IOs to the
root.  One of the issues is that there's no way to tell who originated
a writeback IO from block layer.  Those IOs are usually issued
asynchronously from a task which didn't have anything to do with
actually generating the dirty pages.  The memory subsystem, when
enabled, already keeps track of the ownership of each dirty page and
it's desirable for blkio to piggyback instead of adding its own
per-page tag.

blkio piggybacking on memory is an implementation detail which
preferably should be handled automatically without requiring explicit
userland action.  To achieve that, this patch implements
cgroup_subsys->depends_on which contains the mask of subsystems which
should be enabled together when the subsystem is enabled.

The previous patches already implemented the support for enabled but
invisible subsystems and cgroup_subsys->depends_on can be easily
implemented by updating cgroup_refresh_child_subsys_mask() so that it
calculates cgroup->child_subsys_mask considering
cgroup_subsys->depends_on of the explicitly enabled subsystems.

Documentation/cgroups/unified-hierarchy.txt is updated to explain that
subsystems may not become immediately available after being unused
from userland and that dependency could be a factor in it.  As
subsystems may already keep residual references, this doesn't
significantly change how subsystem rebinding can be used.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>

1 files changed, 48 insertions, 1 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 3a6b77d7ba4a..cd02e99d5d3b 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1037,9 +1037,56 @@ static void cgroup_put(struct cgroup *cgrp)
 	css_put(&cgrp->self);
 }
 
+/**
+ * cgroup_refresh_child_subsys_mask - update child_subsys_mask
+ * @cgrp: the target cgroup
+ *
+ * On the default hierarchy, a subsystem may request other subsystems to be
+ * enabled together through its ->depends_on mask.  In such cases, more
+ * subsystems than specified in "cgroup.subtree_control" may be enabled.
+ *
+ * This function determines which subsystems need to be enabled given the
+ * current @cgrp->subtree_control and records it in
+ * @cgrp->child_subsys_mask.  The resulting mask is always a superset of
+ * @cgrp->subtree_control and follows the usual hierarchy rules.
+ */
 static void cgroup_refresh_child_subsys_mask(struct cgroup *cgrp)
 {
-	cgrp->child_subsys_mask = cgrp->subtree_control;
+	struct cgroup *parent = cgroup_parent(cgrp);
+	unsigned int cur_ss_mask = cgrp->subtree_control;
+	struct cgroup_subsys *ss;
+	int ssid;
+
+	lockdep_assert_held(&cgroup_mutex);
+
+	if (!cgroup_on_dfl(cgrp)) {
+		cgrp->child_subsys_mask = cur_ss_mask;
+		return;
+	}
+
+	while (true) {
+		unsigned int new_ss_mask = cur_ss_mask;
+
+		for_each_subsys(ss, ssid)
+			if (cur_ss_mask & (1 << ssid))
+				new_ss_mask |= ss->depends_on;
+
+		/*
+		 * Mask out subsystems which aren't available.  This can
+		 * happen only if some depended-upon subsystems were bound
+		 * to non-default hierarchies.
+		 */
+		if (parent)
+			new_ss_mask &= parent->child_subsys_mask;
+		else
+			new_ss_mask &= cgrp->root->subsys_mask;
+
+		if (new_ss_mask == cur_ss_mask)
+			break;
+		cur_ss_mask = new_ss_mask;
+	}
+
+	cgrp->child_subsys_mask = cur_ss_mask;
 }
 
 /**