linux-kernel - [PATCH v12 6/9] sched/isolation: Force housekeeping if isolcpus and nohz

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Message-ID: <20250915145920.140180-17-gmonaco@redhat.com>
Date: Mon, 15 Sep 2025 16:59:27 +0200
From: Gabriele Monaco <gmonaco@...hat.com>
To: linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Cc: Gabriele Monaco <gmonaco@...hat.com>,
	Waiman Long <longman@...hat.com>,
	Frederic Weisbecker <frederic@...nel.org>
Subject: [PATCH v12 6/9] 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 (domain) and nohz_full covers all CPUs.

Reviewed-by: Waiman Long <longman@...hat.com>
Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Gabriele Monaco <gmonaco@...hat.com>
---
 kernel/sched/isolation.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index a4cf17b1fab0..3ad0d6df6a0a 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -167,6 +167,29 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
 			}
 		}
 
+		/*
+		 * Check the combination of nohz_full and isolcpus=domain,
+		 * necessary to avoid problems with the timer migration
+		 * hierarchy. managed_irq is ignored by this check since it
+		 * isn't considered in the timer migration logic.
+		 */
+		iter_flags = housekeeping.flags & (HK_FLAG_KERNEL_NOISE | HK_FLAG_DOMAIN);
+		type = find_first_bit(&iter_flags, HK_TYPE_MAX);
+		/*
+		 * Pass the check if none of these flags were previously set or
+		 * are not in the current selection.
+		 */
+		iter_flags = flags & (HK_FLAG_KERNEL_NOISE | HK_FLAG_DOMAIN);
+		first_cpu = (type == HK_TYPE_MAX || !iter_flags) ? 0 :
+			    cpumask_first_and_and(cpu_present_mask,
+				    housekeeping_staging, housekeeping.cpumasks[type]);
+		if (first_cpu >= min(nr_cpu_ids, setup_max_cpus)) {
+			pr_warn("Housekeeping: must include one present CPU "
+				"neither in nohz_full= nor in isolcpus=domain, "
+				"ignoring setting %s\n", str);
+			goto free_housekeeping_staging;
+		}
+
 		iter_flags = flags & ~housekeeping.flags;
 
 		for_each_set_bit(type, &iter_flags, HK_TYPE_MAX)
-- 
2.51.0