linux-kernel - [PATCH v6 4/6] sched/isolation: Force housekeeping if isolcpus and nohz

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Message-ID: <20250530142031.215594-5-gmonaco@redhat.com>
Date: Fri, 30 May 2025 16:20:29 +0200
From: Gabriele Monaco <gmonaco@...hat.com>
To: linux-kernel@...r.kernel.org,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Frederic Weisbecker <frederic@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <longman@...hat.com>
Cc: Gabriele Monaco <gmonaco@...hat.com>
Subject: [PATCH v6 4/6] sched/isolation: Force housekeeping if isolcpus and nohz_full don't leave any

Currently the user can set up isolcpus and nohz_full in such a way that
leaves no housekeeping CPU (i.e. no CPU that is neither domain isolated
nor nohz full). This can be a problem for other subsystems (e.g. the
timer wheel imgration).

Prevent this configuration by invalidating the last setting in case the
union of isolcpus and nohz_full covers all CPUs.

Signed-off-by: Gabriele Monaco <gmonaco@...hat.com>
---
 kernel/sched/isolation.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index 81bc8b329ef17..b3b348427d2b8 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -165,6 +165,18 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
 			}
 		}
 
+		/* Check in combination with the previously set cpumask */
+		type = find_first_bit(&housekeeping.flags, HK_TYPE_MAX);
+		first_cpu = cpumask_first_and_and(cpu_present_mask,
+						  housekeeping_staging,
+						  housekeeping.cpumasks[type]);
+		if (first_cpu >= nr_cpu_ids || first_cpu >= setup_max_cpus) {
+			pr_warn("Housekeeping: must include one present CPU neither "
+				"in nohz_full= nor in isolcpus=, ignoring setting %s\n",
+				str);
+			goto free_housekeeping_staging;
+		}
+
 		iter_flags = flags & ~housekeeping.flags;
 
 		for_each_set_bit(type, &iter_flags, HK_TYPE_MAX)
-- 
2.49.0