softlockup: decouple hung tasks check from softlockup detection

Decoupling allows:

* hung tasks check to happen at very low priority

* hung tasks check and softlockup to be enabled/disabled independently
  at compile and/or run-time

* individual panic settings to be enabled disabled independently
  at compile and/or run-time

* softlockup threshold to be reduced without increasing hung tasks
  poll frequency (hung task check is expensive relative to softlock watchdog)

* hung task check to be zero over-head when disabled at run-time

Signed-off-by: Mandeep Singh Baines <msb@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 198 insertions, 0 deletions

diff --git a/kernel/hung_task.c b/kernel/hung_task.c
new file mode 100644
index 000000000000..ba5a77cad3bb
--- /dev/null
+++ b/kernel/hung_task.c
@@ -0,0 +1,198 @@
+/*
+ * Detect Hung Task
+ *
+ * kernel/hung_task.c - kernel thread for detecting tasks stuck in D state
+ *
+ */
+
+#include <linux/mm.h>
+#include <linux/cpu.h>
+#include <linux/nmi.h>
+#include <linux/init.h>
+#include <linux/delay.h>
+#include <linux/freezer.h>
+#include <linux/kthread.h>
+#include <linux/lockdep.h>
+#include <linux/module.h>
+#include <linux/sysctl.h>
+
+/*
+ * Have a reasonable limit on the number of tasks checked:
+ */
+unsigned long __read_mostly sysctl_hung_task_check_count = 1024;
+
+/*
+ * Zero means infinite timeout - no checking done:
+ */
+unsigned long __read_mostly sysctl_hung_task_timeout_secs = 120;
+static unsigned long __read_mostly hung_task_poll_jiffies;
+
+unsigned long __read_mostly sysctl_hung_task_warnings = 10;
+
+static int __read_mostly did_panic;
+
+static struct task_struct *watchdog_task;
+
+/*
+ * Should we panic (and reboot, if panic_timeout= is set) when a
+ * hung task is detected:
+ */
+unsigned int __read_mostly sysctl_hung_task_panic =
+                                CONFIG_BOOTPARAM_HUNG_TASK_PANIC_VALUE;
+
+static int __init hung_task_panic_setup(char *str)
+{
+        sysctl_hung_task_panic = simple_strtoul(str, NULL, 0);
+
+        return 1;
+}
+__setup("hung_task_panic=", hung_task_panic_setup);
+
+static int
+hung_task_panic(struct notifier_block *this, unsigned long event, void *ptr)
+{
+        did_panic = 1;
+
+        return NOTIFY_DONE;
+}
+
+static struct notifier_block panic_block = {
+        .notifier_call = hung_task_panic,
+};
+
+/*
+ * Returns seconds, approximately.  We don't need nanosecond
+ * resolution, and we don't need to waste time with a big divide when
+ * 2^30ns == 1.074s.
+ */
+static unsigned long get_timestamp(void)
+{
+        int this_cpu = raw_smp_processor_id();
+
+        return cpu_clock(this_cpu) >> 30LL;  /* 2^30 ~= 10^9 */
+}
+
+static void check_hung_task(struct task_struct *t, unsigned long now)
+{
+        unsigned long switch_count = t->nvcsw + t->nivcsw;
+
+        if (t->flags & PF_FROZEN)
+                return;
+
+        if (switch_count != t->last_switch_count || !t->last_switch_timestamp) {
+                t->last_switch_count = switch_count;
+                t->last_switch_timestamp = now;
+                return;
+        }
+        if ((long)(now - t->last_switch_timestamp) <
+                                        sysctl_hung_task_timeout_secs)
+                return;
+        if (!sysctl_hung_task_warnings)
+                return;
+        sysctl_hung_task_warnings--;
+
+        /*
+         * Ok, the task did not get scheduled for more than 2 minutes,
+         * complain:
+         */
+        printk(KERN_ERR "INFO: task %s:%d blocked for more than "
+                        "%ld seconds.\n", t->comm, t->pid,
+                        sysctl_hung_task_timeout_secs);
+        printk(KERN_ERR "\"echo 0 > /proc/sys/kernel/hung_task_timeout_secs\""
+                        " disables this message.\n");
+        sched_show_task(t);
+        __debug_show_held_locks(t);
+
+        t->last_switch_timestamp = now;
+        touch_nmi_watchdog();
+
+        if (sysctl_hung_task_panic)
+                panic("hung_task: blocked tasks");
+}
+
+/*
+ * Check whether a TASK_UNINTERRUPTIBLE does not get woken up for
+ * a really long time (120 seconds). If that happens, print out
+ * a warning.
+ */
+static void check_hung_uninterruptible_tasks(void)
+{
+        int max_count = sysctl_hung_task_check_count;
+        unsigned long now = get_timestamp();
+        struct task_struct *g, *t;
+
+        /*
+         * If the system crashed already then all bets are off,
+         * do not report extra hung tasks:
+         */
+        if (test_taint(TAINT_DIE) || did_panic)
+                return;
+
+        read_lock(&tasklist_lock);
+        do_each_thread(g, t) {
+                if (!--max_count)
+                        goto unlock;
+                /* use "==" to skip the TASK_KILLABLE tasks waiting on NFS */
+                if (t->state == TASK_UNINTERRUPTIBLE)
+                        check_hung_task(t, now);
+        } while_each_thread(g, t);
+ unlock:
+        read_unlock(&tasklist_lock);
+}
+
+static void update_poll_jiffies(void)
+{
+        /* timeout of 0 will disable the watchdog */
+        if (sysctl_hung_task_timeout_secs == 0)
+                hung_task_poll_jiffies = MAX_SCHEDULE_TIMEOUT;
+        else
+                hung_task_poll_jiffies = sysctl_hung_task_timeout_secs * HZ / 2;
+}
+
+/*
+ * Process updating of timeout sysctl
+ */
+int proc_dohung_task_timeout_secs(struct ctl_table *table, int write,
+                                  struct file *filp, void __user *buffer,
+                                  size_t *lenp, loff_t *ppos)
+{
+        int ret;
+
+        ret = proc_doulongvec_minmax(table, write, filp, buffer, lenp, ppos);
+
+        if (ret || !write)
+                goto out;
+
+        update_poll_jiffies();
+
+        wake_up_process(watchdog_task);
+
+ out:
+        return ret;
+}
+
+/*
+ * kthread which checks for tasks stuck in D state
+ */
+static int watchdog(void *dummy)
+{
+        set_user_nice(current, 0);
+        update_poll_jiffies();
+
+        for ( ; ; ) {
+                while (schedule_timeout_interruptible(hung_task_poll_jiffies));
+                check_hung_uninterruptible_tasks();
+        }
+
+        return 0;
+}
+
+static int __init hung_task_init(void)
+{
+        atomic_notifier_chain_register(&panic_notifier_list, &panic_block);
+        watchdog_task = kthread_run(watchdog, NULL, "khungtaskd");
+
+        return 0;
+}
+
+module_init(hung_task_init);