tracing: Add bulk garbage collection of freeing event_trigger_data

The event trigger data requires a full tracepoint_synchronize_unregister()
call before freeing. That call can take 100s of milliseconds to complete.
In order to allow for bulk freeing of the trigger data, it can not call
the tracepoint_synchronize_unregister() for every individual trigger data
being free.

Create a kthread that gets created the first time a trigger data is freed,
and have it use the lockless llist to get the list of data to free, run
the tracepoint_synchronize_unregister() then free everything in the list.

By freeing hundreds of event_trigger_data elements together, it only
requires two runs of the synchronization function, and not hundreds of
runs. This speeds up the operation by orders of magnitude (milliseconds
instead of several seconds).

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Tom Zanussi <zanussi@kernel.org>
Link: https://patch.msgid.link/20251125214032.151674992@kernel.org
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: rostedt

kernel/trace/trace.h

@@ -22,6 +22,7 @@
 #include <linux/ctype.h>
 #include <linux/once_lite.h>
 #include <linux/ftrace_regs.h>
+#include <linux/llist.h>
 
 #include "pid_list.h"
 
@@ -1808,6 +1809,7 @@ struct event_trigger_data {
 	char				*name;
 	struct list_head		named_list;
 	struct event_trigger_data	*named_data;
+	struct llist_node		llist;
 };
 
 /* Avoid typos */

kernel/trace/trace_events_trigger.c

@@ -6,6 +6,7 @@
  */
 
 #include <linux/security.h>
+#include <linux/kthread.h>
 #include <linux/module.h>
 #include <linux/ctype.h>
 #include <linux/mutex.h>
@@ -17,15 +18,63 @@
 static LIST_HEAD(trigger_commands);
 static DEFINE_MUTEX(trigger_cmd_mutex);
 
+static struct task_struct *trigger_kthread;
+static struct llist_head trigger_data_free_list;
+static DEFINE_MUTEX(trigger_data_kthread_mutex);
+
+/* Bulk garbage collection of event_trigger_data elements */
+static int trigger_kthread_fn(void *ignore)
+{
+	struct event_trigger_data *data, *tmp;
+	struct llist_node *llnodes;
+
+	/* Once this task starts, it lives forever */
+	for (;;) {
+		set_current_state(TASK_INTERRUPTIBLE);
+		if (llist_empty(&trigger_data_free_list))
+			schedule();
+
+		__set_current_state(TASK_RUNNING);
+
+		llnodes = llist_del_all(&trigger_data_free_list);
+
+		/* make sure current triggers exit before free */
+		tracepoint_synchronize_unregister();
+
+		llist_for_each_entry_safe(data, tmp, llnodes, llist)
+			kfree(data);
+	}
+
+	return 0;
+}
+
 void trigger_data_free(struct event_trigger_data *data)
 {
 	if (data->cmd_ops->set_filter)
 		data->cmd_ops->set_filter(NULL, data, NULL);
 
-	/* make sure current triggers exit before free */
-	tracepoint_synchronize_unregister();
+	if (unlikely(!trigger_kthread)) {
+		guard(mutex)(&trigger_data_kthread_mutex);
+		/* Check again after taking mutex */
+		if (!trigger_kthread) {
+			struct task_struct *kthread;
+
+			kthread = kthread_create(trigger_kthread_fn, NULL,
+						 "trigger_data_free");
+			if (!IS_ERR(kthread))
+				WRITE_ONCE(trigger_kthread, kthread);
+		}
+	}
+
+	if (!trigger_kthread) {
+		/* Do it the slow way */
+		tracepoint_synchronize_unregister();
+		kfree(data);
+		return;
+	}
 
-	kfree(data);
+	llist_add(&data->llist, &trigger_data_free_list);
+	wake_up_process(trigger_kthread);
 }
 
 static inline void data_ops_trigger(struct event_trigger_data *data,