Re: [PATCH 6/6] Makes procs file writable to move all threads by tgid at once

Serge E. Hallyn
2009-08-03T18:01:58Z

Quoting Ben Blum (bblum@google.com):
...
> +static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
> + struct task_struct *tsk, int guarantee)
> +{
> + struct css_set *oldcg;
> + struct css_set *newcg;
> +
> + /*
> + * get old css_set. we need to take task_lock and refcount it, because
> + * an exiting task can change its css_set to init_css_set and drop its
> + * old one without taking cgroup_mutex.
> + */
> + task_lock(tsk);
> + oldcg = tsk->cgroups;
> + get_css_set(oldcg);
> + task_unlock(tsk);
> + /*
> + * locate or allocate a new css_set for this task. 'guarantee' tells
> + * us whether or not we are sure that a new css_set already exists;
> + * in that case, we are not allowed to fail, as we won't need malloc.
> + */
> + if (guarantee) {
> + /*
> + * our caller promises us that the css_set we want already
> + * exists, so we use find_existing_css_set directly.
> + */
> + struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT];
> + read_lock(&css_set_lock);
> + newcg = find_existing_css_set(oldcg, cgrp, template);
> + BUG_ON(!newcg);
> + get_css_set(newcg);
> + read_unlock(&css_set_lock);
> + } else {
> + might_sleep();

So cgroup_task_migrate() might sleep, but

...

> + down_write(&leader->cgroup_fork_mutex);
> + rcu_read_lock();
> + list_for_each_entry_rcu(tsk, &leader->thread_group, thread_group) {
> + /* leave current thread as it is if it's already there */
> + oldcgrp = task_cgroup(tsk, subsys_id);
> + if (cgrp == oldcgrp)
> + continue;
> + /* we don't care whether these threads are exiting */
> + retval = cgroup_task_migrate(cgrp, oldcgrp, tsk, 1);

Here it is called under rcu_read_lock().

...

> -void cgroup_fork(struct task_struct *child)
> +void cgroup_fork(struct task_struct *child, int clone_flags)
> {
> + if (clone_flags & CLONE_THREAD)
> + down_read(&current->group_leader->cgroup_fork_mutex);
> + else
> + init_rwsem(&child->cgroup_fork_mutex);

I'm also worried about the overhead here on what should be a
fast case, CLONE_THREAD. Have you done any benchmarking of
one thread spawning a bunch of others?

What *exactly* is it we are protecting with cgroup_fork_mutex?
'fork' (as the name implies) is not a good answer, since we should be
protecting data, not code. If it is solely tsk->cgroups, then perhaps
we should in fact try switching to (s?)rcu. Then cgroup_fork() could
just do rcu_read_lock, while cgroup_task_migrate() would make the change
under a spinlock (to protect against concurrent cgroup_task_migrate()s),
and using rcu_assign_pointer to let cgroup_fork() see consistent data
either before or after the update... That might mean that any checks done
before completing the migrate which involve the # of tasks might become
invalidated before the migration completes? Seems acceptable (since
it'll be a small overcharge at most and can be quickly remedied).

-serge