diff options
authorLi Zefan <>2013-01-14 17:23:26 +0800
committerMarcel Ziswiler <>2014-07-25 22:31:06 +0200
commitbfcf5eebc04870bac5dbeaa923e6cce14266a1de (patch)
parent0ef8a43b078dc63d2f1bf69c9d6ee480bdcbb6e7 (diff)
cgroup: remove synchronize_rcu() from cgroup_attach_{task|proc}()
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. # time for ((i=0; i<50; i++)) { echo $$ > /mnt/sub/tasks; echo $$ > /mnt/tasks; } real 0m2.524s user 0m0.008s sys 0m0.004s With this patch: real 0m0.004s user 0m0.004s sys 0m0.000s tj: These synchronize_rcu()s are utterly confused. synchornize_rcu() necessarily has to come between two operations to guarantee that the changes made by the former operation are visible to all rcu readers before proceeding to the latter operation. Here, synchornize_rcu() are at the end of attach operations with nothing beyond it. Its only effect would be delaying completion of write(2) to sysfs tasks/procs files until all rcu readers see the change, which doesn't mean anything. Signed-off-by: Li Zefan <> Signed-off-by: Tejun Heo <> Reported-by: Colin Cross <> Conflicts: kernel/cgroup.c
1 files changed, 0 insertions, 1 deletions
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 54a36fe288f0..9fad3c1616a0 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2164,7 +2164,6 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
* step 5: success! and cleanup
- synchronize_rcu();
retval = 0;