On Wed, Mar 16, 2016 at 07:24:14AM -0700, Tejun Heo wrote:
> So, the problem here is that cpu is using css_offline to tear down the
> css. perf shouldn't have the same problem as it destroys its css from
> css_free. The distinctions among different callbacks evolved over
> time and we need to update the documentation but here are the rules.
>
> css_alloc()
>
> This one is obvious. Alloc and init. The css will become
> visible during css iteration sometime between alloc and
> online.
>
> css_online()
>
> The css is now guaranteed to be visible for css_for_each*()
> iterations. This distinction exists because some controllers
> need to propagate state changes downwards requiring a new css
> to become visible before it inherits the current state from
> the parent. Conversely, there's no reason to use this
> callback if there's no such requirement.
>
> Ex: Freezer which propagates the target state downwards and
> needs a new child to inherit the current state while
> iteratable.
So it looks like sched uses css_online() for no particular reason
either, I've moved all that into css_alloc().
>
> css_offline()
>
> The css is going down and draining usages by refusing new
> css_tryget_online()'s but still guaranteed to be visible
> during css iterations. Controllers which explicitly needs to
> put, say, cache references or need to perform cleanup
> operations while the css is iteratable need to use this
> method; otherwise, no reason to bother with it.
>
> Ex: blkcg pins csses as part of lookup cache which can prevent
> a css from being drained and released, so blkcg uses this call
> back to disable caching for the css.
>
> css_released()
>
> The css is drained and not visible during iteration and will
> be freed after a RCU grace period. Used by controllers which
> cache pointers to csses being drained.
>
> Ex: memcg needs to iterate csses which are being drained and
> its custom iterator implementation caches RCU protected
> pointers to csses. memcg uses this callback to shoot down
> those cached pointers.
>
> css_free()
>
> The css is drained, not visible to iterations, not used by
> anyone, and will be freed immediately.
None of that speaks of where Zombies live, am I to infer that Zombies
pass css_offline() but stall css_released() ?
I don't particularly care about iterating css bits, but I do need my
parent group to still exist, this is now also guaranteed for
css_release(), right? The above documentation also doesn't mention this;
in particular I require that css_release() for any group is not called
before the css_release() of any child group.
The reason I'm interested in css_release() is that there is an RCU grace
period between that and css_free(), so I can kill my call_rcu() from
css_free().
> > So something needs to fundamentally ensure that ->css changes before we
> > go offline the thing.
>
> I could be mistaken but AFAICS there doesn't seem to be anything
> requiring bothering with the more specialized exit methods. Given
> that no css iteration is used and everything is lock protected, the
> patch at the end of this messages should do and seems to work fine
> here. Am I missing something?
Some additional cleanups maybe.. but yes, that seems to work.
Thanks!
---
Subject: sched: Fix/cleanup cgroup teardown/init
The cpu controller hasn't kept up with the various changes in the whole
cgroup initialization / destruction sequence, and commit 2e91fa7f6d45
("cgroup: keep zombies associated with their original cgroups") caused
it to explode.
The reason for this is that zombies do not inhibit css_offline() from
being called, but do stall css_released(). Now we tear down the cfs_rq
structures on css_offline() but zombies can run after that, leading to
use-after-free issues.
The solution is to move the tear-down to css_released(), which
guarantees nobody (including no zombies) is still using our cgroup.
Furthermore, a few simple cleanups are possible too. There doesn't
appear to be any point to us using css_online() (anymore?) so fold that
in css_alloc().
And since cgroup code guarantees an RCU grace period between
css_released() and css_free() we can forgo using call_rcu() and free the
stuff immediately.
Cc: [email protected]
Fixes: 2e91fa7f6d45 ("cgroup: keep zombies associated with their original
cgroups")
Suggested-by: Tejun Heo <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
---
kernel/sched/core.c | 35 ++++++++++++++---------------------
1 file changed, 14 insertions(+), 21 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 04a04e0378fe..bc5076fd0167 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7538,7 +7538,7 @@ void set_curr_task(int cpu, struct task_struct *p)
/* task_group_lock serializes the addition/removal of task groups */
static DEFINE_SPINLOCK(task_group_lock);
-static void free_sched_group(struct task_group *tg)
+static void sched_free_group(struct task_group *tg)
{
free_fair_sched_group(tg);
free_rt_sched_group(tg);
@@ -7564,7 +7564,7 @@ struct task_group *sched_create_group(struct task_group
*parent)
return tg;
err:
- free_sched_group(tg);
+ sched_free_group(tg);
return ERR_PTR(-ENOMEM);
}
@@ -7584,17 +7584,16 @@ void sched_online_group(struct task_group *tg, struct
task_group *parent)
}
/* rcu callback to free various structures associated with a task group */
-static void free_sched_group_rcu(struct rcu_head *rhp)
+static void sched_free_group_rcu(struct rcu_head *rhp)
{
/* now it should be safe to free those cfs_rqs */
- free_sched_group(container_of(rhp, struct task_group, rcu));
+ sched_free_group(container_of(rhp, struct task_group, rcu));
}
-/* Destroy runqueue etc associated with a task group */
void sched_destroy_group(struct task_group *tg)
{
/* wait for possible concurrent references to cfs_rqs complete */
- call_rcu(&tg->rcu, free_sched_group_rcu);
+ call_rcu(&tg->rcu, sched_free_group_rcu);
}
void sched_offline_group(struct task_group *tg)
@@ -8053,31 +8052,26 @@ cpu_cgroup_css_alloc(struct cgroup_subsys_state
*parent_css)
if (IS_ERR(tg))
return ERR_PTR(-ENOMEM);
+ sched_online_group(tg, parent);
+
return &tg->css;
}
-static int cpu_cgroup_css_online(struct cgroup_subsys_state *css)
+static void cpu_cgroup_css_released(struct cgroup_subsys_state *css)
{
struct task_group *tg = css_tg(css);
- struct task_group *parent = css_tg(css->parent);
- if (parent)
- sched_online_group(tg, parent);
- return 0;
+ sched_offline_group(tg);
}
static void cpu_cgroup_css_free(struct cgroup_subsys_state *css)
{
struct task_group *tg = css_tg(css);
- sched_destroy_group(tg);
-}
-
-static void cpu_cgroup_css_offline(struct cgroup_subsys_state *css)
-{
- struct task_group *tg = css_tg(css);
-
- sched_offline_group(tg);
+ /*
+ * Relies on the RCU grace period between css_released() and this.
+ */
+ sched_free_group(tg);
}
static void cpu_cgroup_fork(struct task_struct *task)
@@ -8437,9 +8431,8 @@ static struct cftype cpu_files[] = {
struct cgroup_subsys cpu_cgrp_subsys = {
.css_alloc = cpu_cgroup_css_alloc,
+ .css_released = cpu_cgroup_css_released,
.css_free = cpu_cgroup_css_free,
- .css_online = cpu_cgroup_css_online,
- .css_offline = cpu_cgroup_css_offline,
.fork = cpu_cgroup_fork,
.can_attach = cpu_cgroup_can_attach,
.attach = cpu_cgroup_attach,
