linux-kernel - [PATCH-cgroup 3/5] cgroup/cpuset: Eliminate unncessary sched domains rebuilds in hotplug

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Message-ID: <20240805013019.724300-4-longman@redhat.com>
Date: Sun,  4 Aug 2024 21:30:17 -0400
From: Waiman Long <longman@...hat.com>
To: Tejun Heo <tj@...nel.org>,
	Zefan Li <lizefan.x@...edance.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Koutný <mkoutny@...e.com>,
	Shuah Khan <shuah@...nel.org>
Cc: linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	Chen Ridong <chenridong@...wei.com>,
	Waiman Long <longman@...hat.com>
Subject: [PATCH-cgroup 3/5] cgroup/cpuset: Eliminate unncessary sched domains rebuilds in hotplug

It was found that some hotplug operations may cause multiple
rebuild_sched_domains_locked() calls. Some of those intermediate calls
may use cpuset states not in the final correct form leading to incorrect
sched domain setting.

Fix this problem by using the existing force_rebuild flag to inhibit
immediate rebuild_sched_domains_locked() calls if set and only doing
one final call at the end. Also renaming the force_rebuild flag to
force_sd_rebuild to make its meaning for clear.

Signed-off-by: Waiman Long <longman@...hat.com>
---
 kernel/cgroup/cpuset.c | 33 +++++++++++++++++----------------
 1 file changed, 17 insertions(+), 16 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 7287cecb27d1..e070e391d7a8 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -231,6 +231,13 @@ static cpumask_var_t	isolated_cpus;
 /* List of remote partition root children */
 static struct list_head remote_children;
 
+/*
+ * A flag to force sched domain rebuild at the end of an operation while
+ * inhibiting it in the intermediate stages when set. Currently it is only
+ * set in hotplug code.
+ */
+static bool force_sd_rebuild;
+
 /*
  * Partition root states:
  *
@@ -1467,7 +1474,7 @@ static void update_partition_sd_lb(struct cpuset *cs, int old_prs)
 			clear_bit(CS_SCHED_LOAD_BALANCE, &cs->flags);
 	}
 
-	if (rebuild_domains)
+	if (rebuild_domains && !force_sd_rebuild)
 		rebuild_sched_domains_locked();
 }
 
@@ -1820,7 +1827,7 @@ static void remote_partition_check(struct cpuset *cs, struct cpumask *newmask,
 			remote_partition_disable(child, tmp);
 			disable_cnt++;
 		}
-	if (disable_cnt)
+	if (disable_cnt && !force_sd_rebuild)
 		rebuild_sched_domains_locked();
 }
 
@@ -2425,7 +2432,8 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp,
 	}
 	rcu_read_unlock();
 
-	if (need_rebuild_sched_domains && !(flags & HIER_NO_SD_REBUILD))
+	if (need_rebuild_sched_domains && !(flags & HIER_NO_SD_REBUILD) &&
+	    !force_sd_rebuild)
 		rebuild_sched_domains_locked();
 }
 
@@ -3087,7 +3095,8 @@ static int update_flag(cpuset_flagbits_t bit, struct cpuset *cs,
 	cs->flags = trialcs->flags;
 	spin_unlock_irq(&callback_lock);
 
-	if (!cpumask_empty(trialcs->cpus_allowed) && balance_flag_changed)
+	if (!cpumask_empty(trialcs->cpus_allowed) && balance_flag_changed &&
+	    !force_sd_rebuild)
 		rebuild_sched_domains_locked();
 
 	if (spread_flag_changed)
@@ -4468,11 +4477,9 @@ hotplug_update_tasks(struct cpuset *cs,
 		update_tasks_nodemask(cs);
 }
 
-static bool force_rebuild;
-
 void cpuset_force_rebuild(void)
 {
-	force_rebuild = true;
+	force_sd_rebuild = true;
 }
 
 /**
@@ -4620,15 +4627,9 @@ static void cpuset_handle_hotplug(void)
 		       !cpumask_empty(subpartitions_cpus);
 	mems_updated = !nodes_equal(top_cpuset.effective_mems, new_mems);
 
-	/*
-	 * In the rare case that hotplug removes all the cpus in
-	 * subpartitions_cpus, we assumed that cpus are updated.
-	 */
-	if (!cpus_updated && !cpumask_empty(subpartitions_cpus))
-		cpus_updated = true;
-
 	/* For v1, synchronize cpus_allowed to cpu_active_mask */
 	if (cpus_updated) {
+		cpuset_force_rebuild();
 		spin_lock_irq(&callback_lock);
 		if (!on_dfl)
 			cpumask_copy(top_cpuset.cpus_allowed, &new_cpus);
@@ -4684,8 +4685,8 @@ static void cpuset_handle_hotplug(void)
 	}
 
 	/* rebuild sched domains if cpus_allowed has changed */
-	if (cpus_updated || force_rebuild) {
-		force_rebuild = false;
+	if (force_sd_rebuild) {
+		force_sd_rebuild = false;
 		rebuild_sched_domains_cpuslocked();
 	}
 
-- 
2.43.5