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Subject[PATCH v9 1/4] cgroups: use bitmask to filter for_each_subsys
Date
Add a new macro for_each_subsys_which that allows all enabled cgroup
subsystems to be filtered by a bitmask, such that mask & (1 << ssid)
determines if the subsystem is to be processed in the loop body (where
ssid is the unique id of the subsystem).

Also replace the need_forkexit_callback with two separate bitmasks for
each callback to make (ss->{fork,exit}) checks unnecessary.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
kernel/cgroup.c | 43 ++++++++++++++++++++++++++-----------------
1 file changed, 26 insertions(+), 17 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 29a7b2c..1d69a085 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -175,12 +175,14 @@ static DEFINE_IDR(cgroup_hierarchy_idr);
*/
static u64 css_serial_nr_next = 1;

-/* This flag indicates whether tasks in the fork and exit paths should
+/*
+ * These bitmask flags indicate whether tasks in the fork and exit paths should
* check for fork/exit handlers to call. This avoids us having to do
* extra work in the fork/exit path if none of the subsystems need to
* be called.
*/
-static int need_forkexit_callback __read_mostly;
+static int need_fork_callback __read_mostly;
+static int need_exit_callback __read_mostly;

static struct cftype cgroup_dfl_base_files[];
static struct cftype cgroup_legacy_base_files[];
@@ -409,6 +411,19 @@ static int notify_on_release(const struct cgroup *cgrp)
for ((ssid) = 0; (ssid) < CGROUP_SUBSYS_COUNT && \
(((ss) = cgroup_subsys[ssid]) || true); (ssid)++)

+/**
+ * for_each_subsys_which - filter for_each_subsys with a bitmask
+ * @ss_mask: the bitmask
+ * @ss: the iteration cursor
+ * @ssid: the index of @ss, CGROUP_SUBSYS_COUNT after reaching the end
+ *
+ * The block will only run for cases where the ssid-th bit (1 << ssid) of
+ * mask is set to 1.
+ */
+#define for_each_subsys_which(ss_mask, ss, ssid) \
+ for_each_subsys((ss), (ssid)) \
+ if ((ss_mask) & (1 << (ssid)))
+
/* iterate across the hierarchies */
#define for_each_root(root) \
list_for_each_entry((root), &cgroup_roots, root_list)
@@ -4932,7 +4947,8 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss, bool early)
* init_css_set is in the subsystem's root cgroup. */
init_css_set.subsys[ss->id] = css;

- need_forkexit_callback |= ss->fork || ss->exit;
+ need_fork_callback |= (bool) ss->fork << ss->id;
+ need_exit_callback |= (bool) ss->exit << ss->id;

/* At system boot, before all subsystems have been
* registered, no tasks have been forked, so we don't
@@ -5239,11 +5255,8 @@ void cgroup_post_fork(struct task_struct *child)
* css_set; otherwise, @child might change state between ->fork()
* and addition to css_set.
*/
- if (need_forkexit_callback) {
- for_each_subsys(ss, i)
- if (ss->fork)
- ss->fork(child);
- }
+ for_each_subsys_which(need_fork_callback, ss, i)
+ ss->fork(child);
}

/**
@@ -5287,16 +5300,12 @@ void cgroup_exit(struct task_struct *tsk)
cset = task_css_set(tsk);
RCU_INIT_POINTER(tsk->cgroups, &init_css_set);

- if (need_forkexit_callback) {
- /* see cgroup_post_fork() for details */
- for_each_subsys(ss, i) {
- if (ss->exit) {
- struct cgroup_subsys_state *old_css = cset->subsys[i];
- struct cgroup_subsys_state *css = task_css(tsk, i);
+ /* see cgroup_post_fork() for details */
+ for_each_subsys_which(need_exit_callback, ss, i) {
+ struct cgroup_subsys_state *old_css = cset->subsys[i];
+ struct cgroup_subsys_state *css = task_css(tsk, i);

- ss->exit(css, old_css, tsk);
- }
- }
+ ss->exit(css, old_css, tsk);
}

if (put_cset)
--
2.3.5


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