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@xxxxxxxxxx>
Acked-by: Zefan Li <lizefan@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxx>
---
include/linux/cgroup.h | 22 ++++++++++++++++++++++
kernel/cgroup.c | 11 -----------
kernel/cpuset.c | 9 ++++-----
mm/memcontrol.c | 17 +++++++++++++++--
4 files changed, 41 insertions(+), 18 deletions(-)
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index eb7ca55..916a1e0 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -211,11 +211,33 @@ void css_task_iter_end(struct css_task_iter *it);
* cgroup_taskset_for_each - iterate cgroup_taskset
* @task: the loop cursor
* @tset: taskset to iterate
+ *
+ * @tset may contain multiple tasks and they may belong to multiple
+ * processes. When there are multiple tasks in @tset, if a task of a
+ * process is in @tset, all tasks of the process are in @tset. Also, all
+ * are guaranteed to share the same source and destination csses.
+ *
+ * Iteration is not in any specific order.
*/
#define cgroup_taskset_for_each(task, tset) \
for ((task) = cgroup_taskset_first((tset)); (task); \
(task) = cgroup_taskset_next((tset)))
+/**
+ * cgroup_taskset_for_each_leader - iterate group leaders in a cgroup_taskset
+ * @leader: the loop cursor
+ * @tset: takset to iterate
+ *
+ * Iterate threadgroup leaders of @tset. For single-task migrations, @tset
+ * may not contain any.
+ */
+#define cgroup_taskset_for_each_leader(leader, tset) \
+ for ((leader) = cgroup_taskset_first((tset)); (leader); \
+ (leader) = cgroup_taskset_next((tset))) \
+ if ((leader) != (leader)->group_leader) \
+ ; \
+ else
+
/*
* Inline functions.
*/
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 2cf0f79..0b732dd 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2083,13 +2083,6 @@ static void cgroup_task_migrate(struct cgroup *old_cgrp,
get_css_set(new_cset);
rcu_assign_pointer(tsk->cgroups, new_cset);
-
- /*
- * Use move_tail so that cgroup_taskset_first() still returns the
- * leader after migration. This works because cgroup_migrate()
- * ensures that the dst_cset of the leader is the first on the
- * tset's dst_csets list.
- */
list_move_tail(&tsk->cg_list, &new_cset->mg_tasks);
/*
@@ -2285,10 +2278,6 @@ static int cgroup_migrate(struct cgroup *cgrp, struct task_struct *leader,
if (!cset->mg_src_cgrp)
goto next;
- /*
- * cgroup_taskset_first() must always return the leader.
- * Take care to avoid disturbing the ordering.
- */
list_move_tail(&task->cg_list, &cset->mg_tasks);
if (list_empty(&cset->mg_node))
list_add_tail(&cset->mg_node, &tset.src_csets);
diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 09393f6..e7afde6 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -1485,7 +1485,7 @@ static void cpuset_attach(struct cgroup_subsys_state *css,
/* 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;
@@ -1511,12 +1511,11 @@ static void cpuset_attach(struct cgroup_subsys_state *css,
}
/*
- * 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) {
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6ddaeba..32b6bfd 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4829,7 +4829,7 @@ static int mem_cgroup_can_attach(struct cgroup_subsys_state *css,
{
struct mem_cgroup *memcg = mem_cgroup_from_css(css);
struct mem_cgroup *from;
- struct task_struct *p;
+ struct task_struct *leader, *p;
struct mm_struct *mm;
unsigned long move_flags;
int ret = 0;
@@ -4843,7 +4843,20 @@ static int mem_cgroup_can_attach(struct cgroup_subsys_state *css,
if (!move_flags)
return 0;
- p = cgroup_taskset_first(tset);
+ /*
+ * Multi-process migrations only happen on the default hierarchy
+ * where charge immigration is not used. Perform charge
+ * immigration if @tset contains a leader and whine if there are
+ * multiple.
+ */
+ p = NULL;
+ cgroup_taskset_for_each_leader(leader, tset) {
+ WARN_ON_ONCE(p);
+ p = leader;
+ }
+ if (!p)
+ return 0;
+
from = mem_cgroup_from_task(p);
VM_BUG_ON(from == memcg);
--
2.4.3
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