cpuset: convert cpuset_attach() to use cpumask_var_t

Impact: reduce stack usage

Allocate a global cpumask_var_t at boot, and use it in cpuset_attach(), so
we won't fail cpuset_attach().
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Mike Travis <travis@sgi.com>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/cpuset.c

@@ -1306,6 +1306,9 @@ static int fmeter_getrate(struct fmeter *fmp)
 	return val;
 }
 
+/* Protected by cgroup_lock */
+static cpumask_var_t cpus_attach;
+
 /* Called by cgroups to determine if a cpuset is usable; cgroup_mutex held */
 static int cpuset_can_attach(struct cgroup_subsys *ss,
 			     struct cgroup *cont, struct task_struct *tsk)
@@ -1330,7 +1333,6 @@ static void cpuset_attach(struct cgroup_subsys *ss,
 			  struct cgroup *cont, struct cgroup *oldcont,
 			  struct task_struct *tsk)
 {
-	cpumask_t cpus;
 	nodemask_t from, to;
 	struct mm_struct *mm;
 	struct cpuset *cs = cgroup_cs(cont);
@@ -1338,13 +1340,13 @@ static void cpuset_attach(struct cgroup_subsys *ss,
 	int err;
 
 	if (cs == &top_cpuset) {
-		cpus = cpu_possible_map;
+		cpumask_copy(cpus_attach, cpu_possible_mask);
 	} else {
 		mutex_lock(&callback_mutex);
-		guarantee_online_cpus(cs, &cpus);
+		guarantee_online_cpus(cs, cpus_attach);
 		mutex_unlock(&callback_mutex);
 	}
-	err = set_cpus_allowed_ptr(tsk, &cpus);
+	err = set_cpus_allowed_ptr(tsk, cpus_attach);
 	if (err)
 		return;
 
@@ -1357,7 +1359,6 @@ static void cpuset_attach(struct cgroup_subsys *ss,
 			cpuset_migrate_mm(mm, &from, &to);
 		mmput(mm);
 	}
-
 }
 
 /* The various types of files and directories in a cpuset file system */
@@ -1838,6 +1839,9 @@ int __init cpuset_init(void)
 	if (err < 0)
 		return err;
 
+	if (!alloc_cpumask_var(&cpus_attach, GFP_KERNEL))
+		BUG();
+
 	number_of_cpusets = 1;
 	return 0;
 }