On Wed, Jun 12, 2013 at 09:04:58PM -0700, Tejun Heo wrote: > A css (cgroup_subsys_state) is how each cgroup is represented to a > controller. As such, it can be used in hot paths across the various > subsystems different controllers are associated with. > > One of the common operations is reference counting, which up until now > has been implemented using a global atomic counter and can have > significant adverse impact on scalability. For example, css refcnt > can be gotten and put multiple times by blkcg for each IO request. > For highops configurations which try to do as much per-cpu as > possible, the global frequent refcnting can be very expensive. > > In general, given the various and hugely diverse paths css's end up > being used from, we need to make it cheap and highly scalable. In its > usage, css refcnting isn't very different from module refcnting. > > This patch converts css refcnting to use the recently added > percpu_ref. css_get/tryget/put() directly maps to the matching > percpu_ref operations and the deactivation logic is no longer > necessary as percpu_ref already has refcnt killing. > > The only complication is that as the refcnt is per-cpu, > percpu_ref_kill() in itself doesn't ensure that further tryget > operations will fail, which we need to guarantee before invoking > ->css_offline()'s. This is resolved collecting kill confirmation > using percpu_ref_kill_and_confirm() and initiating the offline phase > of destruction after all css refcnt's are confirmed to be seen as > killed on all CPUs. The previous patches already splitted destruction > into two phases, so percpu_ref_kill_and_confirm() can be hooked up > easily. > > This patch removes css_refcnt() which is used for rcu dereference > sanity check in css_id(). While we can add a percpu refcnt API to ask > the same question, css_id() itself is scheduled to be removed fairly > soon, so let's not bother with it. Just drop the sanity check and use > rcu_dereference_raw() instead. > > v2: - init_cgroup_css() was calling percpu_ref_init() without checking > the return value. This causes two problems - the obvious lack > of error handling and percpu_ref_init() being called from > cgroup_init_subsys() before the allocators are up, which > triggers warnings but doesn't cause actual problems as the > refcnt isn't used for roots anyway. Fix both by moving > percpu_ref_init() to cgroup_create(). > > - The base references were put too early by > percpu_ref_kill_and_confirm() and cgroup_offline_fn() put the > refs one extra time. This wasn't noticeable because css's go > through another RCU grace period before being freed. Update > cgroup_destroy_locked() to grab an extra reference before > killing the refcnts. This problem was noticed by Kent. Reviewed-by: Kent Overstreet <koverstreet@xxxxxxxxxx> _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
