[RFC PATCH v2 10/13] mm/kwatch: add anchor thread for global watchpoints

Global variables have no function whose execution can bound the
watch window. Provide one: a kernel thread sleeps for the configured
duration inside a dedicated noinline function,
kwatch_global_anchor(), and the probe runtime hooks that function
like any other target.

When the duration expires the thread schedules a work item that
tears the session down; the expired flag is cleared under the
control-plane mutex so a stale work item from a previous session
cannot stop a new one.

Signed-off-by: Jinchao Wang <[email protected]>
---
 mm/kwatch/Makefile |  2 +-
 mm/kwatch/anchor.c | 85 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 86 insertions(+), 1 deletion(-)
 create mode 100644 mm/kwatch/anchor.c

diff --git a/mm/kwatch/Makefile b/mm/kwatch/Makefile
index f04673cc5b1c..b196c794619a 100644
--- a/mm/kwatch/Makefile
+++ b/mm/kwatch/Makefile
@@ -1,3 +1,3 @@
 obj-$(CONFIG_KWATCH) += kwatch.o
 
-kwatch-y := deref.o task_ctx.o hwbp.o probe.o
+kwatch-y := deref.o task_ctx.o hwbp.o probe.o anchor.o
diff --git a/mm/kwatch/anchor.c b/mm/kwatch/anchor.c
new file mode 100644
index 000000000000..e87eb5e813ff
--- /dev/null
+++ b/mm/kwatch/anchor.c
@@ -0,0 +1,85 @@
+// SPDX-License-Identifier: GPL-2.0
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/kthread.h>
+#include <linux/wait.h>
+#include <linux/jiffies.h>
+#include <linux/err.h>
+#include <linux/workqueue.h>
+
+#include "kwatch.h"
+
+static DECLARE_WAIT_QUEUE_HEAD(kwatch_anchor_wq);
+static struct task_struct *kwatch_anchor_tsk;
+static bool kwatch_anchor_expired;
+
+bool kwatch_anchor_has_expired(void)
+{
+	return READ_ONCE(kwatch_anchor_expired);
+}
+
+void kwatch_anchor_clear_expired(void)
+{
+	WRITE_ONCE(kwatch_anchor_expired, false);
+}
+
+static void kwatch_auto_stop_handler(struct work_struct *work)
+{
+	kwatch_auto_stop();
+}
+
+static DECLARE_WORK(kwatch_auto_stop_work, kwatch_auto_stop_handler);
+
+noinline void kwatch_global_anchor(unsigned long duration_sec)
+{
+	/* TASK_IDLE: a long timed sleep must not inflate loadavg or trip the
+	 * hung-task detector the way TASK_UNINTERRUPTIBLE would.
+	 */
+	wait_event_idle_timeout(kwatch_anchor_wq, kthread_should_stop(),
+				duration_sec * HZ);
+}
+
+static int kwatch_anchor_thread_fn(void *data)
+{
+	unsigned long duration = (unsigned long)data;
+
+	kwatch_global_anchor(duration);
+
+	if (!kthread_should_stop()) {
+		/* mark before scheduling; cleared under the control mutex */
+		WRITE_ONCE(kwatch_anchor_expired, true);
+		schedule_work(&kwatch_auto_stop_work);
+	}
+
+	while (!kthread_should_stop())
+		schedule_timeout_idle(HZ);
+
+	return 0;
+}
+
+int kwatch_anchor_start(u16 duration)
+{
+	kwatch_anchor_tsk = kthread_run(kwatch_anchor_thread_fn,
+					(void *)(unsigned long)duration,
+					"kwatch_anchor");
+	if (IS_ERR(kwatch_anchor_tsk)) {
+		int ret = PTR_ERR(kwatch_anchor_tsk);
+
+		kwatch_anchor_tsk = NULL;
+		return ret;
+	}
+	return 0;
+}
+
+void kwatch_anchor_stop(void)
+{
+	if (kwatch_anchor_tsk) {
+		kthread_stop(kwatch_anchor_tsk);
+		kwatch_anchor_tsk = NULL;
+	}
+}
+
+void kwatch_anchor_cancel_work(void)
+{
+	cancel_work_sync(&kwatch_auto_stop_work);
+}
-- 
2.53.0