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    SubjectRe: [BUG 2.6.25-rc3] scheduler/hotplug: some processes are dealocked when cpu is set to offline
    On Tue, Mar 04, 2008 at 06:01:07PM +0300, Oleg Nesterov wrote:
    > On 03/04, Gautham R Shenoy wrote:
    > >
    > > So at times, the callback thread is blocked on kthread_stop(k) in
    > > softlockup.c, while other time, it was blocked in
    > > cleanup_workqueue_threads() in workqueue.c.
    >
    > >From another message:
    > >
    > > However, it remains in R< state
    >
    > What about cwq->thread? Was it TASK_RUNNING too?
    >
    > Perhaps, for some reason the task can't get CPU after migrating from
    > the now dead CPU.
    >
    > I can't reproduce this problem on my one cpu P4-ht, perhaps you can
    > try something like the untested/uncompiled patch below?
    >
    > Oleg.
    >
    > --- include/linux/sched.h 2008-02-15 16:59:17.000000000 +0300
    > +++ include/linux/sched.h 2008-03-04 17:44:53.136738605 +0300
    > @@ -1121,6 +1121,7 @@ struct task_struct {
    > /* hung task detection */
    > unsigned long last_switch_timestamp;
    > unsigned long last_switch_count;
    > + unsigned long xxx;
    > #endif
    > /* CPU-specific state of this task */
    > struct thread_struct thread;
    > --- kernel/fork.c 2008-02-15 16:59:17.000000000 +0300
    > +++ kernel/fork.c 2008-03-04 17:45:14.773033839 +0300
    > @@ -1097,6 +1097,7 @@ static struct task_struct *copy_process(
    > #ifdef CONFIG_DETECT_SOFTLOCKUP
    > p->last_switch_count = 0;
    > p->last_switch_timestamp = 0;
    > + p->xxx = 0;
    > #endif
    >
    > #ifdef CONFIG_TASK_XACCT
    > --- kernel/sched.c 2008-02-15 16:59:17.000000000 +0300
    > +++ kernel/sched.c 2008-03-04 17:48:42.308798646 +0300
    > @@ -1291,6 +1291,7 @@ static void enqueue_task(struct rq *rq,
    > sched_info_queued(p);
    > p->sched_class->enqueue_task(rq, p, wakeup);
    > p->se.on_rq = 1;
    > + p->xxx = jiffies | 1;
    > }
    >
    > static void dequeue_task(struct rq *rq, struct task_struct *p, int sleep)
    > @@ -3944,6 +3945,8 @@ need_resched_nonpreemptible:
    > preempt_enable_no_resched();
    > if (unlikely(test_thread_flag(TIF_NEED_RESCHED)))
    > goto need_resched;
    > +
    > + current->xxx = 0;
    > }
    > EXPORT_SYMBOL(schedule);
    >
    > --- kernel/softlockup.c 2008-02-15 16:59:17.000000000 +0300
    > +++ kernel/softlockup.c 2008-03-04 17:49:05.584414763 +0300
    > @@ -174,6 +174,27 @@ static void check_hung_task(struct task_
    > touch_nmi_watchdog();
    > }
    >
    > +static void check_running_task(struct task_struct *t, unsigned long now)
    > +{
    > + if (!sysctl_hung_task_timeout_secs)
    > + return;
    > +

    This function gets called only when t->xxx == 0,
    so the if below doesn't mean much, does it? :)

    > + if (time_before(now, t->xxx + HZ * sysctl_hung_task_timeout_secs)
    > + return;
    > +
    > + printk(KERN_ERR "INFO: task %s:%d can't get CPU for more than "
    > + "%ld seconds.\n", t->comm, t->pid,
    > + sysctl_hung_task_timeout_secs);
    > +
    > + if (!cpus_intersects(t->cpus_allowed, cpu_online_map))
    > + printk(KERN_ERR "bad ->cpus_allowed\n");
    > + if (!cpu_online(task_cpu(t)))
    > + printk(KERN_ERR "bad ->cpu\n");
    > +
    > + sched_show_task(t);
    > + touch_nmi_watchdog();
    > +}
    > +
    > /*
    > * Check whether a TASK_UNINTERRUPTIBLE does not get woken up for
    > * a really long time (120 seconds). If that happens, print out
    > @@ -183,6 +204,7 @@ static void check_hung_uninterruptible_t
    > {
    > int max_count = sysctl_hung_task_check_count;
    > unsigned long now = get_timestamp(this_cpu);
    > + unsigned long jiff = jiffies;
    > struct task_struct *g, *t;
    >
    > /*
    > @@ -192,15 +214,17 @@ static void check_hung_uninterruptible_t
    > if ((tainted & TAINT_DIE) || did_panic)
    > return;
    >
    > - read_lock(&tasklist_lock);
    > + rcu_read_lock();
    > do_each_thread(g, t) {
    > if (!--max_count)
    > goto unlock;
    > if (t->state & TASK_UNINTERRUPTIBLE)
    > check_hung_task(t, now);
    > + if (!t->xxx)
    > + check_running_task(t, jiff);
    > } while_each_thread(g, t);
    > unlock:
    > - read_unlock(&tasklist_lock);
    > + rcu_read_unlock();
    > }
    >
    > /*

    --
    Thanks and Regards
    gautham


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