linux-kernel - [cgroup/for-6.20 PATCH 2/4] cgroup/cpuset: Consistently compute effective_xcpus in update_cpumasks

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Message-ID: <20251225073056.30789-3-longman@redhat.com>
Date: Thu, 25 Dec 2025 02:30:54 -0500
From: Waiman Long <longman@...hat.com>
To: Tejun Heo <tj@...nel.org>,
	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,
	Sun Shaojie <sunshaojie@...inos.cn>,
	Chen Ridong <chenridong@...weicloud.com>,
	Waiman Long <longman@...hat.com>
Subject: [cgroup/for-6.20 PATCH 2/4] cgroup/cpuset: Consistently compute effective_xcpus in update_cpumasks_hier()

Since commit f62a5d39368e ("cgroup/cpuset: Remove remote_partition_check()
& make update_cpumasks_hier() handle remote partition"), the
compute_effective_exclusive_cpumask() helper was extended to
strip exclusive CPUs from siblings when computing effective_xcpus
(cpuset.cpus.exclusive.effective). This helper was later renamed to
compute_excpus() in commit 86bbbd1f33ab ("cpuset: Refactor exclusive
CPU mask computation logic").

This helper is supposed to be used consistently to compute
effective_xcpus. However, there is an exception within the callback
critical section in update_cpumasks_hier() when exclusive_cpus of a
valid partition root is empty. This can cause effective_xcpus value to
differ depending on where exactly it is last computed. Fix this by using
compute_excpus() in this case to give a consistent result.

Signed-off-by: Waiman Long <longman@...hat.com>
---
 kernel/cgroup/cpuset.c | 15 +++++----------
 1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 3d2d28f0fd03..850334dbc36a 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2050,6 +2050,7 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp,
 		struct cpuset *parent = parent_cs(cp);
 		bool remote = is_remote_partition(cp);
 		bool update_parent = false;
+		bool empty_xcpus;
 
 		old_prs = new_prs = cp->partition_root_state;
 
@@ -2160,20 +2161,14 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp,
 			new_prs = cp->partition_root_state;
 		}
 
+		empty_xcpus = cpumask_empty(cp->exclusive_cpus);
 		spin_lock_irq(&callback_lock);
 		cpumask_copy(cp->effective_cpus, tmp->new_cpus);
 		cp->partition_root_state = new_prs;
-		if (!cpumask_empty(cp->exclusive_cpus) && (cp != cs))
+		if (((new_prs > 0) && empty_xcpus) ||
+		    ((cp != cs) && !empty_xcpus))
 			compute_excpus(cp, cp->effective_xcpus);
-
-		/*
-		 * Make sure effective_xcpus is properly set for a valid
-		 * partition root.
-		 */
-		if ((new_prs > 0) && cpumask_empty(cp->exclusive_cpus))
-			cpumask_and(cp->effective_xcpus,
-				    cp->cpus_allowed, parent->effective_xcpus);
-		else if (new_prs < 0)
+		if (new_prs < 0)
 			reset_partition_data(cp);
 		spin_unlock_irq(&callback_lock);
 
-- 
2.52.0