[RFC] hung task: check specific tasks for long uninterruptible sleep state

[Lingutla Chandrasekhar]

khungtask by default monitors all tasks for long unterruptible sleep.
This change introduces a sysctl option, /proc/sys/kernel/
hung_task_selective_monitoring, to enable monitoring selected tasks.
If this sysctl option is enabled then only the tasks with
/proc/$PID/hang_detection_enabled set are to be monitored,
otherwise all tasks are monitored as default case.

Some tasks may intentionally moves to uninterruptable sleep state,
which shouldn't leads to khungtask panics, as those are recoverable
hungs. So to avoid false hung reports, add an option to select tasks
to be monitored and report/panic them only.

Signed-off-by: Imran Khan <kimran@xxxxxxxxxxxxxx>
Signed-off-by: Lingutla Chandrasekhar <clingutla@xxxxxxxxxxxxxx>

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 9d357b2ea6cb..810f9a5e209e 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2733,6 +2733,52 @@ static int proc_tgid_io_accounting(struct seq_file *m, struct pid_namespace *ns,
 }
 #endif /* CONFIG_TASK_IO_ACCOUNTING */
 
+#ifdef CONFIG_DETECT_HUNG_TASK
+static ssize_t proc_hung_task_detection_enabled_read(struct file *file,
+				char __user *buf, size_t count, loff_t *ppos)
+{
+	struct task_struct *task = get_proc_task(file_inode(file));
+	char buffer[PROC_NUMBUF];
+	size_t len;
+	bool hang_detection_enabled;
+
+	if (!task)
+		return -ESRCH;
+	hang_detection_enabled = task->hang_detection_enabled;
+	put_task_struct(task);
+
+	len = snprintf(buffer, sizeof(buffer), "%d\n", hang_detection_enabled);
+
+	return simple_read_from_buffer(buf, sizeof(buffer), ppos, buffer, len);
+}
+
+static ssize_t proc_hung_task_detection_enabled_write(struct file *file,
+			const char __user *buf, size_t count, loff_t *ppos)
+{
+	struct task_struct *task;
+	bool hang_detection_enabled;
+	int rv;
+
+	rv = kstrtobool_from_user(buf, count, &hang_detection_enabled);
+	if (rv < 0)
+		return rv;
+
+	task = get_proc_task(file_inode(file));
+	if (!task)
+		return -ESRCH;
+	task->hang_detection_enabled = hang_detection_enabled;
+	put_task_struct(task);
+
+	return count;
+}
+
+static const struct file_operations proc_hung_task_detection_enabled_operations = {
+	.read		= proc_hung_task_detection_enabled_read,
+	.write		= proc_hung_task_detection_enabled_write,
+	.llseek		= generic_file_llseek,
+};
+#endif
+
 #ifdef CONFIG_USER_NS
 static int proc_id_map_open(struct inode *inode, struct file *file,
 	const struct seq_operations *seq_ops)
@@ -2976,6 +3022,10 @@ static const struct pid_entry tgid_base_stuff[] = {
 #ifdef CONFIG_HARDWALL
 	ONE("hardwall",   S_IRUGO, proc_pid_hardwall),
 #endif
+#ifdef CONFIG_DETECT_HUNG_TASK
+	REG("hang_detection_enabled", 0666,
+		proc_hung_task_detection_enabled_operations),
+#endif
 #ifdef CONFIG_USER_NS
 	REG("uid_map",    S_IRUGO|S_IWUSR, proc_uid_map_operations),
 	REG("gid_map",    S_IRUGO|S_IWUSR, proc_gid_map_operations),
@@ -3367,6 +3417,10 @@ static const struct pid_entry tid_base_stuff[] = {
 #ifdef CONFIG_HARDWALL
 	ONE("hardwall",   S_IRUGO, proc_pid_hardwall),
 #endif
+#ifdef CONFIG_DETECT_HUNG_TASK
+	REG("hang_detection_enabled", 0666,
+		proc_hung_task_detection_enabled_operations),
+#endif
 #ifdef CONFIG_USER_NS
 	REG("uid_map",    S_IRUGO|S_IWUSR, proc_uid_map_operations),
 	REG("gid_map",    S_IRUGO|S_IWUSR, proc_gid_map_operations),
diff --git a/include/linux/sched.h b/include/linux/sched.h
index fdf74f27acf1..1280df0a6708 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -767,6 +767,7 @@ struct task_struct {
 #endif
 #ifdef CONFIG_DETECT_HUNG_TASK
 	unsigned long			last_switch_count;
+	bool				hang_detection_enabled;
 #endif
 	/* Filesystem information: */
 	struct fs_struct		*fs;
diff --git a/include/linux/sched/sysctl.h b/include/linux/sched/sysctl.h
index d6a18a3839cc..09d9f1c65fd2 100644
--- a/include/linux/sched/sysctl.h
+++ b/include/linux/sched/sysctl.h
@@ -11,6 +11,7 @@ extern int	     sysctl_hung_task_check_count;
 extern unsigned int  sysctl_hung_task_panic;
 extern unsigned long sysctl_hung_task_timeout_secs;
 extern int sysctl_hung_task_warnings;
+extern int sysctl_hung_task_selective_monitoring;
 extern int proc_dohung_task_timeout_secs(struct ctl_table *table, int write,
 					 void __user *buffer,
 					 size_t *lenp, loff_t *ppos);
diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 751593ed7c0b..321bcfa2e34a 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -20,12 +20,21 @@
 #include <linux/sched/debug.h>
 
 #include <trace/events/sched.h>
+#include <linux/sched/sysctl.h>
 
 /*
  * The number of tasks checked:
  */
 int __read_mostly sysctl_hung_task_check_count = PID_MAX_LIMIT;
 
+/*
+ * Selective monitoring of hung tasks.
+ *
+ * if set to 1, khungtaskd only monitors tasks with 'hang_detection_enabled'
+ * value set, else monitors all tasks.
+ */
+int sysctl_hung_task_selective_monitoring;
+
 /*
  * Limit number of tasks checked in a batch.
  *
@@ -187,7 +196,10 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
 		}
 		/* use "==" to skip the TASK_KILLABLE tasks waiting on NFS */
 		if (t->state == TASK_UNINTERRUPTIBLE)
-			check_hung_task(t, timeout);
+			/* Check for selective monitoring */
+			if (!sysctl_hung_task_selective_monitoring ||
+			    t->hang_detection_enabled)
+				check_hung_task(t, timeout);
 	}
  unlock:
 	rcu_read_unlock();
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index d9c31bc2eaea..30cc108ce2cc 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1093,6 +1093,16 @@ static struct ctl_table kern_table[] = {
 		.proc_handler	= proc_dointvec_minmax,
 		.extra1		= &neg_one,
 	},
+	{
+		.procname	= "hung_task_selective_monitoring",
+		.data		= &sysctl_hung_task_selective_monitoring,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= &zero,
+		.extra2		= &one,
+	},
+
 #endif
 #ifdef CONFIG_RT_MUTEXES
 	{
-- 
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