linux-kernel - [tip:sched/core] cpusets, hotplug: Implement cpuset tree traversal in a helper function

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Message-ID: <tip-b451c38428c5682348b6f661d767e358d1d70c41@git.kernel.org>
Date:	Wed, 20 Jun 2012 03:45:35 -0700
From:	"tip-bot for Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...nel.org,
	torvalds@...ux-foundation.org, a.p.zijlstra@...llo.nl,
	srivatsa.bhat@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
	tglx@...utronix.de
Subject: [tip:sched/core] cpusets, hotplug:
  Implement cpuset tree traversal in a helper function

Commit-ID:  b451c38428c5682348b6f661d767e358d1d70c41
Gitweb:     http://git.kernel.org/tip/b451c38428c5682348b6f661d767e358d1d70c41
Author:     Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
AuthorDate: Thu, 24 May 2012 19:46:41 +0530
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Mon, 18 Jun 2012 11:45:04 +0200

cpusets, hotplug: Implement cpuset tree traversal in a helper function

At present, the functions that deal with cpusets during CPU/Mem hotplug
are quite messy, since a lot of the functionality is mixed up without clear
separation. And this takes a toll on optimization as well. For example,
the function cpuset_update_active_cpus() is called on both CPU offline and CPU
online events; and it invokes scan_for_empty_cpusets(), which makes sense
only for CPU offline events. And hence, the current code ends up unnecessarily
traversing the cpuset tree during CPU online also.

As a first step towards cleaning up those functions, encapsulate the cpuset
tree traversal in a helper function, so as to facilitate upcoming changes.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/20120524141635.3692.893.stgit@srivatsabhat.in.ibm.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/cpuset.c |   36 +++++++++++++++++++++++++++---------
 1 files changed, 27 insertions(+), 9 deletions(-)

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 77abe1a..e0b29bb 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -1990,6 +1990,32 @@ static void remove_tasks_in_empty_cpuset(struct cpuset *cs)
 }
 
 /*
+ * Helper function to traverse cpusets.
+ * It can be used to walk the cpuset tree from top to bottom, completing
+ * one layer before dropping down to the next (thus always processing a
+ * node before any of its children).
+ */
+static struct cpuset *cpuset_next(struct list_head *queue)
+{
+	struct cpuset *cp;
+	struct cpuset *child;	/* scans child cpusets of cp */
+	struct cgroup *cont;
+
+	if (list_empty(queue))
+		return NULL;
+
+	cp = list_first_entry(queue, struct cpuset, stack_list);
+	list_del(queue->next);
+	list_for_each_entry(cont, &cp->css.cgroup->children, sibling) {
+		child = cgroup_cs(cont);
+		list_add_tail(&child->stack_list, queue);
+	}
+
+	return cp;
+}
+
+
+/*
  * Walk the specified cpuset subtree and look for empty cpusets.
  * The tasks of such cpuset must be moved to a parent cpuset.
  *
@@ -2008,19 +2034,11 @@ static void scan_for_empty_cpusets(struct cpuset *root)
 {
 	LIST_HEAD(queue);
 	struct cpuset *cp;	/* scans cpusets being updated */
-	struct cpuset *child;	/* scans child cpusets of cp */
-	struct cgroup *cont;
 	static nodemask_t oldmems;	/* protected by cgroup_mutex */
 
 	list_add_tail((struct list_head *)&root->stack_list, &queue);
 
-	while (!list_empty(&queue)) {
-		cp = list_first_entry(&queue, struct cpuset, stack_list);
-		list_del(queue.next);
-		list_for_each_entry(cont, &cp->css.cgroup->children, sibling) {
-			child = cgroup_cs(cont);
-			list_add_tail(&child->stack_list, &queue);
-		}
+	while ((cp = cpuset_next(&queue)) != NULL) {
 
 		/* Continue past cpusets with all cpus, mems online */
 		if (cpumask_subset(cp->cpus_allowed, cpu_active_mask) &&
--
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