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Commit ce9dbe24 authored by Mandeep Singh Baines's avatar Mandeep Singh Baines Committed by Ingo Molnar
Browse files

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: default avatarMandeep Singh Baines <msb@google.com>
Suggested-by: default avatarFrédéric Weisbecker <fweisbec@gmail.com>
Acked-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 5e54f598

kernel/hung_task.c

+37 −2
Original line number Diff line number Diff line
@@ -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);