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Subject[PATCH 2/2] softlockup: check all tasks in hung_task
Instead of checking only hung_task_check_count tasks, all tasks are checked.
hung_task_check_count is still used to put an upper bound on the critical
section. Every hung_task_check_count checks, the critical section is
refreshed. 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.

Frédéric Weisbecker (fweisbec@gmail.com) wrote:
>
> Instead of having this arbitrary limit of tasks, why not just
> lurk the need_resched() and then schedule if it needs too.
>
> I know that sounds a bit racy, because you will have to release the
> tasklist_lock and
> a lot of things can happen in the task list until you become resched.
> But you can do a get_task_struct() on g and t before your thread is
> going to sleep and then put them
> when it is awaken.
> Perhaps some tasks will disappear or be appended in the list before g
> and t, but that doesn't really matter:
> if they disappear, they didn't lockup, and if they were appended, they
> are not enough cold to be analyzed :-)
>
> This way you can drop the arbitrary limit of task number given by the user....
>
> Frederic.
>

Signed-off-by: Mandeep Singh Baines <msb@google.com>
---
kernel/hung_task.c | 28 ++++++++++++++++++++++++++--
1 files changed, 26 insertions(+), 2 deletions(-)
diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index a841db3..1c8c9f9 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -109,6 +109,25 @@ 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 check_hung_rcu_refresh(struct task_struct *g, struct task_struct *t)
+{
+ get_task_struct(g);
+ get_task_struct(t);
+ rcu_read_unlock();
+ if (need_resched())
+ schedule();
+ 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
@@ -129,8 +148,13 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)

rcu_read_lock();
do_each_thread(g, t) {
- if (!--max_count)
- goto unlock;
+ if (sysctl_hung_task_check_count && !(max_count--)) {
+ max_count = sysctl_hung_task_check_count;
+ check_hung_rcu_refresh(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);
--
1.5.4.5
--
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