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.

Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Waiman Long <longman@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://patch.msgid.link/20251120145653.296659-6-gmonaco@redhat.com

diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index a4cf17b1fab062f536c7f4f47c35f0e209fd25d6..3ad0d6df6a0a213f894946fb1d51b537d48cd607 100644 (file)
--- 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)