[PATCH v8 10/27] mm/ksw: support CPU hotplug

Jinchao Wang
2025-11-10T16:37:28Z

Register CPU online/offline callbacks via cpuhp_setup_state_nocalls()
so stack watches are installed/removed dynamically as CPUs come online
or go offline.

When a new CPU comes online, register a hardware breakpoint for the holder,
avoiding races with watch_on()/watch_off() that may run on another CPU. The
watch address will be updated the next time watch_on() is called.

Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
---
mm/kstackwatch/watch.c | 52 ++++++++++++++++++++++++++++++++++++++++++
1 file changed, 52 insertions(+)

diff --git a/mm/kstackwatch/watch.c b/mm/kstackwatch/watch.c
index f922b4164be5..99184f63d7e3 100644
--- a/mm/kstackwatch/watch.c
+++ b/mm/kstackwatch/watch.c
@@ -85,6 +85,48 @@ static void ksw_watch_on_local_cpu(void *info)
WARN(ret, "fail to reinstall HWBP on CPU%d ret %d", cpu, ret);
}

+static int ksw_watch_cpu_online(unsigned int cpu)
+{
+ struct perf_event_attr attr;
+ struct ksw_watchpoint *wp;
+ call_single_data_t *csd;
+ struct perf_event *bp;
+
+ mutex_lock(&all_wp_mutex);
+ list_for_each_entry(wp, &all_wp_list, list) {
+ attr = wp->attr;
+ attr.bp_addr = (u64)&holder;
+ bp = perf_event_create_kernel_counter(&attr, cpu, NULL,
+ ksw_watch_handler, wp);
+ if (IS_ERR(bp)) {
+ pr_warn("%s failed to create watch on CPU %d: %ld\n",
+ __func__, cpu, PTR_ERR(bp));
+ continue;
+ }
+
+ per_cpu(*wp->event, cpu) = bp;
+ csd = per_cpu_ptr(wp->csd, cpu);
+ INIT_CSD(csd, ksw_watch_on_local_cpu, wp);
+ }
+ mutex_unlock(&all_wp_mutex);
+ return 0;
+}
+
+static int ksw_watch_cpu_offline(unsigned int cpu)
+{
+ struct ksw_watchpoint *wp;
+ struct perf_event *bp;
+
+ mutex_lock(&all_wp_mutex);
+ list_for_each_entry(wp, &all_wp_list, list) {
+ bp = per_cpu(*wp->event, cpu);
+ if (bp)
+ unregister_hw_breakpoint(bp);
+ }
+ mutex_unlock(&all_wp_mutex);
+ return 0;
+}
+
static void ksw_watch_update(struct ksw_watchpoint *wp, ulong addr, u16 len)
{
call_single_data_t *csd;
@@ -206,6 +248,16 @@ int ksw_watch_init(void)
if (ret <= 0)
return -EBUSY;

+ ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
+ "kstackwatch:online",
+ ksw_watch_cpu_online,
+ ksw_watch_cpu_offline);
+ if (ret < 0) {
+ ksw_watch_free();
+ pr_err("Failed to register CPU hotplug notifier\n");
+ return ret;
+ }
+
return 0;
}

--
2.43.0