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>
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@ -232,11 +232,33 @@ void css_task_iter_end(struct css_task_iter *it);
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* cgroup_taskset_for_each - iterate cgroup_taskset
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* @task: the loop cursor
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* @tset: taskset to iterate
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*
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* @tset may contain multiple tasks and they may belong to multiple
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* processes. When there are multiple tasks in @tset, if a task of a
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* process is in @tset, all tasks of the process are in @tset. Also, all
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* are guaranteed to share the same source and destination csses.
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*
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* Iteration is not in any specific order.
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*/
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#define cgroup_taskset_for_each(task, tset) \
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for ((task) = cgroup_taskset_first((tset)); (task); \
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(task) = cgroup_taskset_next((tset)))
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/**
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* cgroup_taskset_for_each_leader - iterate group leaders in a cgroup_taskset
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* @leader: the loop cursor
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* @tset: takset to iterate
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*
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* Iterate threadgroup leaders of @tset. For single-task migrations, @tset
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* may not contain any.
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*/
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#define cgroup_taskset_for_each_leader(leader, tset) \
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for ((leader) = cgroup_taskset_first((tset)); (leader); \
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(leader) = cgroup_taskset_next((tset))) \
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if ((leader) != (leader)->group_leader) \
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; \
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else
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/*
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* Inline functions.
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*/
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@ -2217,13 +2217,6 @@ static void cgroup_task_migrate(struct cgroup *old_cgrp,
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get_css_set(new_cset);
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rcu_assign_pointer(tsk->cgroups, new_cset);
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/*
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* Use move_tail so that cgroup_taskset_first() still returns the
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* leader after migration. This works because cgroup_migrate()
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* ensures that the dst_cset of the leader is the first on the
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* tset's dst_csets list.
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*/
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list_move_tail(&tsk->cg_list, &new_cset->mg_tasks);
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/*
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@ -2419,10 +2412,6 @@ static int cgroup_migrate(struct cgroup *cgrp, struct task_struct *leader,
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if (!cset->mg_src_cgrp)
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goto next;
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/*
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* cgroup_taskset_first() must always return the leader.
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* Take care to avoid disturbing the ordering.
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*/
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list_move_tail(&task->cg_list, &cset->mg_tasks);
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if (list_empty(&cset->mg_node))
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list_add_tail(&cset->mg_node, &tset.src_csets);
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@ -1488,7 +1488,7 @@ static void cpuset_attach(struct cgroup_subsys_state *css,
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/* static buf protected by cpuset_mutex */
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static nodemask_t cpuset_attach_nodemask_to;
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struct task_struct *task;
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struct task_struct *leader = cgroup_taskset_first(tset);
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struct task_struct *leader;
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struct cpuset *cs = css_cs(css);
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struct cpuset *oldcs = cpuset_attach_old_cs;
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@ -1514,12 +1514,11 @@ static void cpuset_attach(struct cgroup_subsys_state *css,
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}
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/*
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* Change mm, possibly for multiple threads in a threadgroup. This
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* is expensive and may sleep and should be moved outside migration
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* path proper.
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* Change mm for all threadgroup leaders. This is expensive and may
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* sleep and should be moved outside migration path proper.
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*/
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cpuset_attach_nodemask_to = cs->effective_mems;
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if (thread_group_leader(leader)) {
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cgroup_taskset_for_each_leader(leader, tset) {
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struct mm_struct *mm = get_task_mm(leader);
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if (mm) {
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@ -4828,7 +4828,7 @@ static int mem_cgroup_can_attach(struct cgroup_subsys_state *css,
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{
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struct mem_cgroup *memcg = mem_cgroup_from_css(css);
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struct mem_cgroup *from;
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struct task_struct *p;
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struct task_struct *leader, *p;
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struct mm_struct *mm;
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unsigned long move_flags;
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int ret = 0;
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@ -4842,7 +4842,20 @@ static int mem_cgroup_can_attach(struct cgroup_subsys_state *css,
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if (!move_flags)
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return 0;
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p = cgroup_taskset_first(tset);
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/*
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* Multi-process migrations only happen on the default hierarchy
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* where charge immigration is not used. Perform charge
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* immigration if @tset contains a leader and whine if there are
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* multiple.
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*/
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p = NULL;
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cgroup_taskset_for_each_leader(leader, tset) {
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WARN_ON_ONCE(p);
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p = leader;
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}
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if (!p)
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return 0;
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from = mem_cgroup_from_task(p);
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VM_BUG_ON(from == memcg);
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