1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3  * Detect Hung Task
4  *
5  * kernel/hung_task.c - kernel thread for detecting tasks stuck in D state
6  *
7  */
8 
9 #include <linux/mm.h>
10 #include <linux/cpu.h>
11 #include <linux/nmi.h>
12 #include <linux/init.h>
13 #include <linux/delay.h>
14 #include <linux/freezer.h>
15 #include <linux/kthread.h>
16 #include <linux/lockdep.h>
17 #include <linux/export.h>
18 #include <linux/panic_notifier.h>
19 #include <linux/sysctl.h>
20 #include <linux/suspend.h>
21 #include <linux/utsname.h>
22 #include <linux/sched/signal.h>
23 #include <linux/sched/debug.h>
24 #include <linux/sched/sysctl.h>
25 
26 #include <trace/events/sched.h>
27 
28 /*
29  * The number of tasks checked:
30  */
31 int __read_mostly sysctl_hung_task_check_count = PID_MAX_LIMIT;
32 
33 /*
34  * Limit number of tasks checked in a batch.
35  *
36  * This value controls the preemptibility of khungtaskd since preemption
37  * is disabled during the critical section. It also controls the size of
38  * the RCU grace period. So it needs to be upper-bound.
39  */
40 #define HUNG_TASK_LOCK_BREAK (HZ / 10)
41 
42 /*
43  * Zero means infinite timeout - no checking done:
44  */
45 unsigned long __read_mostly sysctl_hung_task_timeout_secs = CONFIG_DEFAULT_HUNG_TASK_TIMEOUT;
46 
47 /*
48  * Zero (default value) means use sysctl_hung_task_timeout_secs:
49  */
50 unsigned long __read_mostly sysctl_hung_task_check_interval_secs;
51 
52 int __read_mostly sysctl_hung_task_warnings = 10;
53 
54 static int __read_mostly did_panic;
55 static bool hung_task_show_lock;
56 static bool hung_task_call_panic;
57 static bool hung_task_show_all_bt;
58 
59 static struct task_struct *watchdog_task;
60 
61 #ifdef CONFIG_SMP
62 /*
63  * Should we dump all CPUs backtraces in a hung task event?
64  * Defaults to 0, can be changed via sysctl.
65  */
66 unsigned int __read_mostly sysctl_hung_task_all_cpu_backtrace;
67 #endif /* CONFIG_SMP */
68 
69 /*
70  * Should we panic (and reboot, if panic_timeout= is set) when a
71  * hung task is detected:
72  */
73 unsigned int __read_mostly sysctl_hung_task_panic =
74 				CONFIG_BOOTPARAM_HUNG_TASK_PANIC_VALUE;
75 
76 static int
hung_task_panic(struct notifier_block * this,unsigned long event,void * ptr)77 hung_task_panic(struct notifier_block *this, unsigned long event, void *ptr)
78 {
79 	did_panic = 1;
80 
81 	return NOTIFY_DONE;
82 }
83 
84 static struct notifier_block panic_block = {
85 	.notifier_call = hung_task_panic,
86 };
87 
check_hung_task(struct task_struct * t,unsigned long timeout)88 static void check_hung_task(struct task_struct *t, unsigned long timeout)
89 {
90 	unsigned long switch_count = t->nvcsw + t->nivcsw;
91 
92 	/*
93 	 * Ensure the task is not frozen.
94 	 * Also, skip vfork and any other user process that freezer should skip.
95 	 */
96 	if (unlikely(t->flags & (PF_FROZEN | PF_FREEZER_SKIP)))
97 	    return;
98 
99 	/*
100 	 * When a freshly created task is scheduled once, changes its state to
101 	 * TASK_UNINTERRUPTIBLE without having ever been switched out once, it
102 	 * musn't be checked.
103 	 */
104 	if (unlikely(!switch_count))
105 		return;
106 
107 	if (switch_count != t->last_switch_count) {
108 		t->last_switch_count = switch_count;
109 		t->last_switch_time = jiffies;
110 		return;
111 	}
112 	if (time_is_after_jiffies(t->last_switch_time + timeout * HZ))
113 		return;
114 
115 	trace_sched_process_hang(t);
116 
117 	if (sysctl_hung_task_panic) {
118 		console_verbose();
119 		hung_task_show_lock = true;
120 		hung_task_call_panic = true;
121 	}
122 
123 	/*
124 	 * Ok, the task did not get scheduled for more than 2 minutes,
125 	 * complain:
126 	 */
127 	if (sysctl_hung_task_warnings) {
128 		if (sysctl_hung_task_warnings > 0)
129 			sysctl_hung_task_warnings--;
130 		pr_err("INFO: task %s:%d blocked for more than %ld seconds.\n",
131 		       t->comm, t->pid, (jiffies - t->last_switch_time) / HZ);
132 		pr_err("      %s %s %.*s\n",
133 			print_tainted(), init_utsname()->release,
134 			(int)strcspn(init_utsname()->version, " "),
135 			init_utsname()->version);
136 		pr_err("\"echo 0 > /proc/sys/kernel/hung_task_timeout_secs\""
137 			" disables this message.\n");
138 		sched_show_task(t);
139 		hung_task_show_lock = true;
140 
141 		if (sysctl_hung_task_all_cpu_backtrace)
142 			hung_task_show_all_bt = true;
143 	}
144 
145 	touch_nmi_watchdog();
146 }
147 
148 /*
149  * To avoid extending the RCU grace period for an unbounded amount of time,
150  * periodically exit the critical section and enter a new one.
151  *
152  * For preemptible RCU it is sufficient to call rcu_read_unlock in order
153  * to exit the grace period. For classic RCU, a reschedule is required.
154  */
rcu_lock_break(struct task_struct * g,struct task_struct * t)155 static bool rcu_lock_break(struct task_struct *g, struct task_struct *t)
156 {
157 	bool can_cont;
158 
159 	get_task_struct(g);
160 	get_task_struct(t);
161 	rcu_read_unlock();
162 	cond_resched();
163 	rcu_read_lock();
164 	can_cont = pid_alive(g) && pid_alive(t);
165 	put_task_struct(t);
166 	put_task_struct(g);
167 
168 	return can_cont;
169 }
170 
171 /*
172  * Check whether a TASK_UNINTERRUPTIBLE does not get woken up for
173  * a really long time (120 seconds). If that happens, print out
174  * a warning.
175  */
check_hung_uninterruptible_tasks(unsigned long timeout)176 static void check_hung_uninterruptible_tasks(unsigned long timeout)
177 {
178 	int max_count = sysctl_hung_task_check_count;
179 	unsigned long last_break = jiffies;
180 	struct task_struct *g, *t;
181 
182 	/*
183 	 * If the system crashed already then all bets are off,
184 	 * do not report extra hung tasks:
185 	 */
186 	if (test_taint(TAINT_DIE) || did_panic)
187 		return;
188 
189 	hung_task_show_lock = false;
190 	rcu_read_lock();
191 	for_each_process_thread(g, t) {
192 		if (!max_count--)
193 			goto unlock;
194 		if (time_after(jiffies, last_break + HUNG_TASK_LOCK_BREAK)) {
195 			if (!rcu_lock_break(g, t))
196 				goto unlock;
197 			last_break = jiffies;
198 		}
199 		/* use "==" to skip the TASK_KILLABLE tasks waiting on NFS */
200 		if (READ_ONCE(t->__state) == TASK_UNINTERRUPTIBLE)
201 			check_hung_task(t, timeout);
202 	}
203  unlock:
204 	rcu_read_unlock();
205 	if (hung_task_show_lock)
206 		debug_show_all_locks();
207 
208 	if (hung_task_show_all_bt) {
209 		hung_task_show_all_bt = false;
210 		trigger_all_cpu_backtrace();
211 	}
212 
213 	if (hung_task_call_panic)
214 		panic("hung_task: blocked tasks");
215 }
216 
hung_timeout_jiffies(unsigned long last_checked,unsigned long timeout)217 static long hung_timeout_jiffies(unsigned long last_checked,
218 				 unsigned long timeout)
219 {
220 	/* timeout of 0 will disable the watchdog */
221 	return timeout ? last_checked - jiffies + timeout * HZ :
222 		MAX_SCHEDULE_TIMEOUT;
223 }
224 
225 /*
226  * Process updating of timeout sysctl
227  */
proc_dohung_task_timeout_secs(struct ctl_table * table,int write,void * buffer,size_t * lenp,loff_t * ppos)228 int proc_dohung_task_timeout_secs(struct ctl_table *table, int write,
229 				  void *buffer, size_t *lenp, loff_t *ppos)
230 {
231 	int ret;
232 
233 	ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
234 
235 	if (ret || !write)
236 		goto out;
237 
238 	wake_up_process(watchdog_task);
239 
240  out:
241 	return ret;
242 }
243 
244 static atomic_t reset_hung_task = ATOMIC_INIT(0);
245 
reset_hung_task_detector(void)246 void reset_hung_task_detector(void)
247 {
248 	atomic_set(&reset_hung_task, 1);
249 }
250 EXPORT_SYMBOL_GPL(reset_hung_task_detector);
251 
252 static bool hung_detector_suspended;
253 
hungtask_pm_notify(struct notifier_block * self,unsigned long action,void * hcpu)254 static int hungtask_pm_notify(struct notifier_block *self,
255 			      unsigned long action, void *hcpu)
256 {
257 	switch (action) {
258 	case PM_SUSPEND_PREPARE:
259 	case PM_HIBERNATION_PREPARE:
260 	case PM_RESTORE_PREPARE:
261 		hung_detector_suspended = true;
262 		break;
263 	case PM_POST_SUSPEND:
264 	case PM_POST_HIBERNATION:
265 	case PM_POST_RESTORE:
266 		hung_detector_suspended = false;
267 		break;
268 	default:
269 		break;
270 	}
271 	return NOTIFY_OK;
272 }
273 
274 /*
275  * kthread which checks for tasks stuck in D state
276  */
watchdog(void * dummy)277 static int watchdog(void *dummy)
278 {
279 	unsigned long hung_last_checked = jiffies;
280 
281 	set_user_nice(current, 0);
282 
283 	for ( ; ; ) {
284 		unsigned long timeout = sysctl_hung_task_timeout_secs;
285 		unsigned long interval = sysctl_hung_task_check_interval_secs;
286 		long t;
287 
288 		if (interval == 0)
289 			interval = timeout;
290 		interval = min_t(unsigned long, interval, timeout);
291 		t = hung_timeout_jiffies(hung_last_checked, interval);
292 		if (t <= 0) {
293 			if (!atomic_xchg(&reset_hung_task, 0) &&
294 			    !hung_detector_suspended)
295 				check_hung_uninterruptible_tasks(timeout);
296 			hung_last_checked = jiffies;
297 			continue;
298 		}
299 		schedule_timeout_interruptible(t);
300 	}
301 
302 	return 0;
303 }
304 
hung_task_init(void)305 static int __init hung_task_init(void)
306 {
307 	atomic_notifier_chain_register(&panic_notifier_list, &panic_block);
308 
309 	/* Disable hung task detector on suspend */
310 	pm_notifier(hungtask_pm_notify, 0);
311 
312 	watchdog_task = kthread_run(watchdog, NULL, "khungtaskd");
313 
314 	return 0;
315 }
316 subsys_initcall(hung_task_init);
317