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

Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
---
 mm/kstackwatch/watch.c | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/mm/kstackwatch/watch.c b/mm/kstackwatch/watch.c
index 14549e02faf1..795e779792da 100644
--- a/mm/kstackwatch/watch.c
+++ b/mm/kstackwatch/watch.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
+#include <linux/cpuhotplug.h>
 #include <linux/hw_breakpoint.h>
 #include <linux/irqflags.h>
 #include <linux/perf_event.h>
@@ -61,6 +62,32 @@ static void ksw_watch_on_local_cpu(void *data)
 	}
 }
 
+static int ksw_cpu_online(unsigned int cpu)
+{
+	struct perf_event *bp;
+
+	bp = perf_event_create_kernel_counter(&watch_attr, cpu, NULL,
+					      ksw_watch_handler, NULL);
+	if (IS_ERR(bp)) {
+		pr_err("Failed to create watch on CPU %d: %ld\n", cpu,
+		       PTR_ERR(bp));
+		return PTR_ERR(bp);
+	}
+
+	per_cpu(*watch_events, cpu) = bp;
+	per_cpu(watch_csd, cpu) = CSD_INIT(ksw_watch_on_local_cpu, NULL);
+	return 0;
+}
+
+static int ksw_cpu_offline(unsigned int cpu)
+{
+	struct perf_event *bp = per_cpu(*watch_events, cpu);
+
+	if (bp)
+		unregister_hw_breakpoint(bp);
+	return 0;
+}
+
 static void __ksw_watch_target(ulong addr, u16 len)
 {
 	int cpu;
@@ -117,6 +144,15 @@ int ksw_watch_init(void)
 		return ret;
 	}
 
+	ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
+					"kstackwatch:online", ksw_cpu_online,
+					ksw_cpu_offline);
+	if (ret < 0) {
+		unregister_wide_hw_breakpoint(watch_events);
+		pr_err("Failed to register CPU hotplug notifier\n");
+		return ret;
+	}
+
 	return 0;
 }
 
-- 
2.43.0