cgroup attach task - slogging cpu

[anjana vk <anjanvk12-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

[-- Attachment #1: Type: text/plain, Size: 1651 bytes --]

Hi Oleg, All

I saw an issue of CPU slogging posted in
https://lkml.org/lkml/2013/8/28/283, and require your valuable
suggestion on a very similar issue I am facing.

We are facing the issue of cpu slogging in the function
    cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk,
bool threadgroup)
in the do-while loop which iterates over the threadgroup.

 rcu_read_lock();
 do {
 struct task_and_cgroup ent;

 /* @tsk either already exited or can't exit until the end */
 if (tsk->flags & PF_EXITING)
 continue;

 /* as per above, nr_threads may decrease, but not increase. */
 BUG_ON(i >= group_size);
 ent.task = tsk;
 ent.cgrp = task_cgroup_from_root(tsk, root);
 /* nothing to do if this task is already in the cgroup */
 if (ent.cgrp == cgrp)
 continue;
 /*
 * saying GFP_ATOMIC has no effect here because we did prealloc
 * earlier, but it's good form to communicate our expectations.
 */
 retval = flex_array_put(group, i, &ent, GFP_ATOMIC);
 BUG_ON(retval != 0);
 i++;

 if (!threadgroup)
 break;
 } while_each_thread(leader, tsk);

Problem Observed:
In this loop, in case of a single thread, and argument “bool
threadgroup” as “false” and
if(ent.cgrp == cgrp) is true
we will continue to the next thread instead of breaking out of the loop.

Possible Solution and Doubt:
When a break condition was added as shown in the patch attached, the
cpu sluggishness was not occurring.
Can you please provide your suggestions, if this is the right way to
fix the above mentioned issue.
Also, if a fix is already in for this, can you please guide me to that.

Thanks and Regards
Anjana

[-- Attachment #2: cgroup_singe_thread_attach.patch --]
[-- Type: application/octet-stream, Size: 737 bytes --]

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 1f53387..cae8416 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2002,7 +2002,9 @@ static int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk,
 		ent.task = tsk;
 		ent.cgrp = task_cgroup_from_root(tsk, root);
 		/* nothing to do if this task is already in the cgroup */
-		if (ent.cgrp == cgrp)
+		if (ent.cgrp == cgrp && !threadgroup)
+			break;
+		else if(ent.cgrp == cgrp)
 			continue;
 		/*
 		 * saying GFP_ATOMIC has no effect here because we did prealloc
@@ -2199,7 +2201,6 @@ retry_find_task:
 	ret = cgroup_attach_task(cgrp, tsk, threadgroup);
 
 	threadgroup_unlock(tsk);
-
 	put_task_struct(tsk);
 out_unlock_cgroup:
 	mutex_unlock(&cgroup_mutex);