linux-kernel - [PATCH v12 8/9] cpuset: Don't rebuild sched domains if cpu changes in non-partition root

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Message-Id: <1535380884-31308-9-git-send-email-longman@redhat.com>
Date:   Mon, 27 Aug 2018 10:41:23 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, kernel-team@...com, pjt@...gle.com,
        luto@...capital.net, Mike Galbraith <efault@....de>,
        torvalds@...ux-foundation.org, Roman Gushchin <guro@...com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Waiman Long <longman@...hat.com>
Subject: [PATCH v12 8/9] cpuset: Don't rebuild sched domains if cpu changes in non-partition root

With the cpuset v1, any changes to the list of allowable CPUs in a cpuset
may cause changes in the sched domain configuration depending on the
load balancing states and the cpu lists of its parent and its children.

With cpuset v2 (on default hierarchy), there are more restrictions
on how the load balancing state of a cpuset can change. As a result,
only changes made in a partition root will cause possible changes to
the corresponding sched domain. CPU changes to any of the non-partition
root cpusets will not cause changes in the sched domain configuration.
As a result, we don't need to call rebuild_sched_domains_locked()
for changes in a non-partition root cpuset saving precious cpu cycles.

Signed-off-by: Waiman Long <longman@...hat.com>
---
 kernel/cgroup/cpuset.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 0e41b62..d8970b4 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -986,11 +986,15 @@ static void update_cpumasks_hier(struct cpuset *cs, struct cpumask *new_cpus)
 		update_tasks_cpumask(cp);
 
 		/*
-		 * If the effective cpumask of any non-empty cpuset is changed,
-		 * we need to rebuild sched domains.
+		 * On legacy hierarchy, if the effective cpumask of any non-
+		 * empty cpuset is changed, we need to rebuild sched domains.
+		 * On default hierarchy, the cpuset needs to be a partition
+		 * root as well.
 		 */
 		if (!cpumask_empty(cp->cpus_allowed) &&
-		    is_sched_load_balance(cp))
+		    is_sched_load_balance(cp) &&
+		   (!cgroup_subsys_on_dfl(cpuset_cgrp_subsys) ||
+		    is_partition_root(cp)))
 			need_rebuild_sched_domains = true;
 
 		rcu_read_lock();
-- 
1.8.3.1