Hey, Li. On Mon, Jan 14, 2013 at 05:23:26PM +0800, Li Zefan wrote: > These 2 syncronize_rcu()s make attaching a task to a cgroup > quite slow, and it can't be ignored in some situations. > > A real case from Colin Cross: Android uses cgroups heavily to > manage thread priorities, putting threads in a background group > with reduced cpu.shares when they are not visible to the user, > and in a foreground group when they are. Some RPCs from foreground > threads to background threads will temporarily move the background > thread into the foreground group for the duration of the RPC. > This results in many calls to cgroup_attach_task. > > In cgroup_attach_task() it's task->cgroups that is protected by RCU, > and put_css_set() calls kfree_rcu() to free it. > > If we remove this synchronize_rcu(), there can be threads in RCU-read > sections accessing their old cgroup via current->cgroups with > concurrent rmdir operation, but this is safe. Yeah, these synchronize_rcu()s are utterly confused. synchornize_rcu() necessarily have to come between two operations to guarantee that the changes made before it are visible to all rcu readers before proceeding beyond it. Here, synchornize_rcu() are at the end of attach operations with nothing beyond it. Duh.... its only effect would be delaying completion of write(2) to sysfs tasks/procs files until all rcu readers see the change, which is utterly irrelevant. It doesn't mean anything. Applying to cgroup/for-3.9. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
