softlockup: check all tasks in hung_task

Impact: extend the scope of hung-task checks

Changed the default value of hung_task_check_count to PID_MAX_LIMIT.
hung_task_batch_count added to put an upper bound on the critical
section. Every hung_task_batch_count checks, the rcu lock is never
held for a too long time.

Keeping the critical section small minimizes time preemption is disabled
and keeps rcu grace periods small.

To prevent following a stale pointer, get_task_struct is called on g and t.
To verify that g and t have not been unhashed while outside the critical
section, the task states are checked.

The design was proposed by Frédéric Weisbecker.

Signed-off-by: Mandeep Singh Baines <msb@google.com>
Suggested-by: Frédéric Weisbecker <fweisbec@gmail.com>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index ba8ccd43296362a68d70e1f542ece29d84d22883..481ca8b5c2bc02f6236dada0796b6394b94e554c 100644 (file)
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -17,9 +17,18 @@
 #include <linux/sysctl.h>
 
 /*
- * Have a reasonable limit on the number of tasks checked:
+ * The number of tasks checked:
  */
-unsigned long __read_mostly sysctl_hung_task_check_count = 1024;
+unsigned long __read_mostly sysctl_hung_task_check_count = PID_MAX_LIMIT;
+
+/*
+ * Limit number of tasks checked in a batch.
+ *
+ * This value controls the preemptibility of khungtaskd since preemption
+ * is disabled during the critical section. It also controls the size of
+ * the RCU grace period. So it needs to be upper-bound.
+ */
+#define HUNG_TASK_BATCHING 1024
 
 /*
  * Zero means infinite timeout - no checking done:
@@ -109,6 +118,24 @@ static void check_hung_task(struct task_struct *t, unsigned long now,
                panic("hung_task: blocked tasks");
 }
 
+/*
+ * To avoid extending the RCU grace period for an unbounded amount of time,
+ * periodically exit the critical section and enter a new one.
+ *
+ * For preemptible RCU it is sufficient to call rcu_read_unlock in order
+ * exit the grace period. For classic RCU, a reschedule is required.
+ */
+static void rcu_lock_break(struct task_struct *g, struct task_struct *t)
+{
+       get_task_struct(g);
+       get_task_struct(t);
+       rcu_read_unlock();
+       cond_resched();
+       rcu_read_lock();
+       put_task_struct(t);
+       put_task_struct(g);
+}
+
 /*
  * Check whether a TASK_UNINTERRUPTIBLE does not get woken up for
  * a really long time (120 seconds). If that happens, print out
@@ -117,6 +144,7 @@ static void check_hung_task(struct task_struct *t, unsigned long now,
 static void check_hung_uninterruptible_tasks(unsigned long timeout)
 {
        int max_count = sysctl_hung_task_check_count;
+       int batch_count = HUNG_TASK_BATCHING;
        unsigned long now = get_timestamp();
        struct task_struct *g, *t;
 
@@ -131,6 +159,13 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
        do_each_thread(g, t) {
                if (!--max_count)
                        goto unlock;
+               if (!--batch_count) {
+                       batch_count = HUNG_TASK_BATCHING;
+                       rcu_lock_break(g, t);
+                       /* Exit if t or g was unhashed during refresh. */
+                       if (t->state == TASK_DEAD || g->state == TASK_DEAD)
+                               goto unlock;
+               }
                /* use "==" to skip the TASK_KILLABLE tasks waiting on NFS */
                if (t->state == TASK_UNINTERRUPTIBLE)
                        check_hung_task(t, now, timeout);