Re: [PATCH v14 7/7] timers: Exclude isolated cpus from timer migration

[Frederic Weisbecker <frederic@kernel.org>]

Le Wed, Nov 12, 2025 at 05:18:26PM +0100, Gabriele Monaco a écrit :
> 
> 
> On Wed, 2025-11-12 at 17:04 +0100, Frederic Weisbecker wrote:
> > Le Tue, Nov 04, 2025 at 11:47:39AM +0100, Gabriele Monaco a écrit :
> > 
> > 
> > This duplicates a lot tmigr_isolated_exclude_cpumask().
> > Would this work?
> > 
> > static int __init tmigr_init_isolation(void)
> > {
> > 	cpumask_var_t cpumask __free(free_cpumask_var) = CPUMASK_VAR_NULL;
> > 
> > 	static_branch_enable(&tmigr_exclude_isolated);
> > 
> > 	if (!housekeeping_enabled(HK_TYPE_DOMAIN))
> > 		return 0;
> > 	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
> > 		return -ENOMEM;
> > 
> > 	cpumask_andnot(cpumask, cpu_possible_mask,
> > housekeeping_cpumask(HK_TYPE_DOMAIN));
> > 
> > 	return tmigr_isolated_exclude_cpumask(cpumask);
> > }
> > 
> > If so please add my Reviewed-by to the next version.
> 
> Good point, it should work, I'm going to test it tomorrow.
> 
> What comes to my mind is that tmigr_isolated_exclude_cpumask expects the hotplug
> lock to be held, so I should probably just hold it before calling it:
> 
> static int __init tmigr_init_isolation(void)
> {
> 	cpumask_var_t cpumask __free(free_cpumask_var) = CPUMASK_VAR_NULL;
> 
> 	static_branch_enable(&tmigr_exclude_isolated);
> 
> 	if (!housekeeping_enabled(HK_TYPE_DOMAIN))
> 		return 0;
> 	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
> 		return -ENOMEM;
> 
> 	cpumask_andnot(cpumask, cpu_possible_mask,
> housekeeping_cpumask(HK_TYPE_DOMAIN));
> +	guard(cpus_read_lock)();
> 	return tmigr_isolated_exclude_cpumask(cpumask);
> }

Well it doesn't make much sense to lock hotplug at this stage.

How about:

diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
index e2f9a4637d7b..6ca870f1b951 100644
--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -1539,17 +1539,7 @@ static void tmigr_cpu_unisolate(struct work_struct *ignored)
 	tmigr_set_cpu_available(smp_processor_id());
 }
 
-/**
- * tmigr_isolated_exclude_cpumask - Exclude given CPUs from hierarchy
- * @exclude_cpumask: the cpumask to be excluded from timer migration hierarchy
- *
- * This function can be called from cpuset code to provide the new set of
- * isolated CPUs that should be excluded from the hierarchy.
- * Online CPUs not present in exclude_cpumask but already excluded are brought
- * back to the hierarchy.
- * Functions to isolate/unisolate need to be called locally and can sleep.
- */
-int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
+static int __tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
 {
 	struct work_struct __percpu *works __free(free_percpu) =
 		alloc_percpu(struct work_struct);
@@ -1557,8 +1547,6 @@ int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
 	cpumask_var_t cpumask_isol __free(free_cpumask_var) = CPUMASK_VAR_NULL;
 	int cpu;
 
-	lockdep_assert_cpus_held();
-
 	if (!alloc_cpumask_var(&cpumask_isol, GFP_KERNEL))
 		return -ENOMEM;
 	if (!alloc_cpumask_var(&cpumask_unisol, GFP_KERNEL))
@@ -1604,40 +1592,36 @@ int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
 	return 0;
 }
 
+/**
+ * tmigr_isolated_exclude_cpumask - Exclude given CPUs from hierarchy
+ * @exclude_cpumask: the cpumask to be excluded from timer migration hierarchy
+ *
+ * This function can be called from cpuset code to provide the new set of
+ * isolated CPUs that should be excluded from the hierarchy.
+ * Online CPUs not present in exclude_cpumask but already excluded are brought
+ * back to the hierarchy.
+ * Functions to isolate/unisolate need to be called locally and can sleep.
+ */
+int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
+{
+	lockdep_assert_cpus_held();
+	return __tmigr_isolated_exclude_cpumask(exclude_cpumask);
+}
+
 static int __init tmigr_init_isolation(void)
 {
-	struct work_struct __percpu *works __free(free_percpu) =
-		alloc_percpu(struct work_struct);
 	cpumask_var_t cpumask __free(free_cpumask_var) = CPUMASK_VAR_NULL;
-	int cpu;
 
 	static_branch_enable(&tmigr_exclude_isolated);
+
 	if (!housekeeping_enabled(HK_TYPE_DOMAIN))
 		return 0;
 	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
 		return -ENOMEM;
-	if (!works)
-		return -ENOMEM;
-	cpumask_andnot(cpumask, tmigr_available_cpumask,
-		       housekeeping_cpumask(HK_TYPE_DOMAIN));
-	cpumask_and(cpumask, cpumask, housekeeping_cpumask(HK_TYPE_KERNEL_NOISE));
-	/* Never disable the tick CPU, see tmigr_is_isolated for details */
-	for_each_cpu(cpu, cpumask) {
-		if (!tick_nohz_cpu_hotpluggable(cpu)) {
-			cpumask_clear_cpu(cpu, cpumask);
-			break;
-		}
-	}
-	for_each_cpu(cpu, cpumask) {
-		struct work_struct *work = per_cpu_ptr(works, cpu);
 
-		INIT_WORK(work, tmigr_cpu_isolate);
-		schedule_work_on(cpu, work);
-	}
-	for_each_cpu(cpu, cpumask)
-		flush_work(per_cpu_ptr(works, cpu));
+	cpumask_andnot(cpumask, cpu_possible_mask, housekeeping_cpumask(HK_TYPE_DOMAIN));
 
-	return 0;
+	return __tmigr_isolated_exclude_cpumask(cpumask);
 }
 
 static void tmigr_init_group(struct tmigr_group *group, unsigned int lvl,