cgroup: use bitmask to filter for_each_subsys

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.

tj: add a short comment for "if (!CGROUP_SUBSYS_COUNT)".

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 26d1cea7929f1da98d1f80d4aa94fea9f6bf3c4b..c5588c438448389ba67a4a008286c4e489ecd5d7 100644 (file)
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -490,6 +490,8 @@ static inline void cgroup_threadgroup_change_end(struct task_struct *tsk)
 
 #else  /* CONFIG_CGROUPS */
 
+#define CGROUP_SUBSYS_COUNT 0
+
 static inline void cgroup_threadgroup_change_begin(struct task_struct *tsk) {}
 static inline void cgroup_threadgroup_change_end(struct task_struct *tsk) {}
 

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 0fd5227958fed86165c994efbf5c13a831f5f7ad..3a973519068ff5fa98989f43608fcd1ff9aa21c1 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -178,12 +178,13 @@ 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
- * 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.
+/*
+ * These bitmask flags indicate whether tasks in the fork and exit paths have
+ * fork/exit handlers to call. This avoids us having to do extra work in the
+ * fork/exit path to check which subsystems have fork/exit callbacks.
  */
-static int need_forkexit_callback __read_mostly;
+static unsigned long have_fork_callback __read_mostly;
+static unsigned long have_exit_callback __read_mostly;
 
 static struct cftype cgroup_dfl_base_files[];
 static struct cftype cgroup_legacy_base_files[];
@@ -412,6 +413,24 @@ 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: the iteration cursor
+ * @ssid: the index of @ss, CGROUP_SUBSYS_COUNT after reaching the end
+ * @ss_maskp: a pointer to the bitmask
+ *
+ * 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, ssid, ss_maskp)                      \
+       if (!CGROUP_SUBSYS_COUNT) /* to avoid spurious gcc warning */   \
+               ;                                                       \
+       else                                                            \
+               for_each_set_bit(ssid, ss_maskp, CGROUP_SUBSYS_COUNT)   \
+                       if (((ss) = cgroup_subsys[ssid]) && false)      \
+                               break;                                  \
+                       else
+
 /* iterate across the hierarchies */
 #define for_each_root(root)                                            \
        list_for_each_entry((root), &cgroup_roots, root_list)
@@ -4914,7 +4933,8 @@ 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;
+       have_fork_callback |= (bool)ss->fork << ss->id;
+       have_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
@@ -5225,11 +5245,8 @@ 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(ss, i, &have_fork_callback)
+               ss->fork(child);
 }
 
 /**
@@ -5273,16 +5290,12 @@ 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(ss, i, &have_exit_callback) {
+               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)