[PATCH RFC 11/12] sched/isolation: Implement SMT sibling auto-isolation and safety check

[Qiliang Yuan <realwujing@gmail.com>]

Implement SMT sibling auto-isolation logic via 'smt_aware_mode' sysfs
node. When enabled, writing to housekeeping masks will automatically
ensure that all siblings of a physical core stay together in the same
isolation state.

Also implement a safety check in housekeeping_store() to ensure that
at least one online CPU remains as housekeeping, preventing the user
from accidentally isolating all CPUs and locking up the system.

Signed-off-by: Qiliang Yuan <realwujing@gmail.com>
Signed-off-by: Qiliang Yuan <yuanql9@chinatelecom.cn>
---
 kernel/sched/isolation.c | 72 ++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 67 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index d9ce26cf31fe..30798e790b9f 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -13,6 +13,7 @@
 #include <linux/sysfs.h>
 #include <linux/slab.h>
 #include <linux/ctype.h>
+#include <linux/topology.h>
 #include "sched.h"
 
 enum hk_flags {
@@ -40,6 +41,30 @@ struct housekeeping {
 };
 
 static struct housekeeping housekeeping;
+static bool housekeeping_smt_aware;
+
+static ssize_t smt_aware_show(struct kobject *kobj,
+			     struct kobj_attribute *attr, char *buf)
+{
+	return sprintf(buf, "%d\n", housekeeping_smt_aware);
+}
+
+static ssize_t smt_aware_store(struct kobject *kobj,
+			      struct kobj_attribute *attr,
+			      const char *buf, size_t count)
+{
+	bool val;
+
+	if (kstrtobool(buf, &val))
+		return -EINVAL;
+
+	housekeeping_smt_aware = val;
+
+	return count;
+}
+
+static struct kobj_attribute smt_aware_attr =
+	__ATTR(smt_aware_mode, 0644, smt_aware_show, smt_aware_store);
 
 bool housekeeping_enabled(enum hk_type type)
 {
@@ -164,11 +189,40 @@ static ssize_t housekeeping_store(struct kobject *kobject,
 	if (err)
 		goto out_free;
 
-	if (cpumask_empty(new_mask)) {
+	/* Safety check: must have at least one online CPU for housekeeping */
+	if (!cpumask_intersects(new_mask, cpu_online_mask)) {
 		err = -EINVAL;
 		goto out_free;
 	}
 
+	if (housekeeping_smt_aware) {
+		int cpu, sibling;
+		cpumask_var_t tmp_mask;
+
+		if (!alloc_cpumask_var(&tmp_mask, GFP_KERNEL)) {
+			err = -ENOMEM;
+			goto out_free;
+		}
+
+		cpumask_copy(tmp_mask, new_mask);
+		for_each_cpu(cpu, tmp_mask) {
+			for_each_cpu(sibling, topology_sibling_cpumask(cpu)) {
+				if (!cpumask_test_cpu(sibling, tmp_mask)) {
+					/* SMT sibling should stay grouped */
+					cpumask_clear_cpu(cpu, new_mask);
+					break;
+				}
+			}
+		}
+		free_cpumask_var(tmp_mask);
+
+		/* Re-check after SMT sync */
+		if (!cpumask_intersects(new_mask, cpu_online_mask)) {
+			err = -EINVAL;
+			goto out_free;
+		}
+	}
+
 	mutex_lock(&housekeeping_mutex);
 	
 	if (!housekeeping.cpumasks[type]) {
@@ -230,12 +284,20 @@ static int __init housekeeping_sysfs_init(void)
 	housekeeping_attr_ptr[j] = NULL;
 
 	ret = sysfs_create_group(housekeeping_kobj, &housekeeping_attr_group);
-	if (ret) {
-		kobject_put(housekeeping_kobj);
-		return ret;
-	}
+	if (ret)
+		goto err_group;
+
+	ret = sysfs_create_file(housekeeping_kobj, &smt_aware_attr.attr);
+	if (ret)
+		goto err_file;
 
 	return 0;
+
+err_file:
+	sysfs_remove_group(housekeeping_kobj, &housekeeping_attr_group);
+err_group:
+	kobject_put(housekeeping_kobj);
+	return ret;
 }
 late_initcall(housekeeping_sysfs_init);
 

-- 
2.51.0