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softlockup: check all tasks in hung_task

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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>
This commit is contained in:
Mandeep Singh Baines 2009-02-04 20:35:48 -08:00 committed by Ingo Molnar
parent 5e54f5986a
commit ce9dbe244b
1 changed files with 37 additions and 2 deletions
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39
kernel/hung_task.c
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@ -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);