cgroup, memcg, cpuset: implement cgroup_taskset_for_each_leader()
It wasn't explicitly documented but, when a process is being migrated,
cpuset and memcg depend on cgroup_taskset_first() returning the
threadgroup leader; however, this approach is somewhat ghetto and
would no longer work for the planned multi-process migration.
This patch introduces explicit cgroup_taskset_for_each_leader() which
iterates over only the threadgroup leaders and replaces
cgroup_taskset_first() usages for accessing the leader with it.
This prepares both memcg and cpuset for multi-process migration. This
patch also updates the documentation for cgroup_taskset_for_each() to
clarify the iteration rules and removes comments mentioning task
ordering in tasksets.
v2: A previous patch which added threadgroup leader test was dropped.
Patch updated accordingly.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 0b361a0..e4d9999 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -1488,7 +1488,7 @@
/* static buf protected by cpuset_mutex */
static nodemask_t cpuset_attach_nodemask_to;
struct task_struct *task;
- struct task_struct *leader = cgroup_taskset_first(tset);
+ struct task_struct *leader;
struct cpuset *cs = css_cs(css);
struct cpuset *oldcs = cpuset_attach_old_cs;
@@ -1514,12 +1514,11 @@
}
/*
- * Change mm, possibly for multiple threads in a threadgroup. This
- * is expensive and may sleep and should be moved outside migration
- * path proper.
+ * Change mm for all threadgroup leaders. This is expensive and may
+ * sleep and should be moved outside migration path proper.
*/
cpuset_attach_nodemask_to = cs->effective_mems;
- if (thread_group_leader(leader)) {
+ cgroup_taskset_for_each_leader(leader, tset) {
struct mm_struct *mm = get_task_mm(leader);
if (mm) {