Re: floppy.c soft lockup

Oleg Nesterov
2007-06-02T12:30:47Z

On 06/01, Mark Hounschell wrote:
>
> Oleg Nesterov wrote:
> > On 06/01, Mark Hounschell wrote:
> >>
> >> Ok the prctl never returned. I just replaced the ioctl with it and added
> >> a printf before and after. I only get the one before. The thread is hung
> >> at this point just as if I'd done the ioctl?
> >
> > Thanks. So we can rule out floppy.c. flush_scheduled_work/flush_workqueue
> > is broken by this RT application. Imho, this is not the kernel problem.
> >
> > Now I am very sure that the initial suspect was correct: cpu starvation.
> > I can cook a debug patch to be 100% sure tomorrow, which kernel version is
> > most convenient to you?
> >
>
> 2.6.22-rc3 is fine thanks.

Please try this patch, it should dump some debug info when flush_workqueue()
hangs (after 30 seconds). You can use it with or without the previous patch
I sent. Please wait for a couple of minutes to collect more info.

Oleg.

--- OLD/kernel/sched.c~TST 2007-04-05 12:20:35.000000000 +0400
+++ OLD/kernel/sched.c 2007-06-02 15:41:53.000000000 +0400
@@ -4177,6 +4177,20 @@ struct task_struct *idle_task(int cpu)
return cpu_rq(cpu)->idle;
}

+struct task_struct *get_cpu_curr(int cpu)
+{
+ unsigned long flags;
+ struct task_struct *curr;
+ struct rq *rq = cpu_rq(cpu);
+
+ spin_lock_irqsave(&rq->lock, flags);
+ curr = rq->curr;
+ get_task_struct(curr);
+ spin_unlock_irqrestore(&rq->lock, flags);
+
+ return curr;
+}
+
/**
* find_process_by_pid - find a process with a matching PID value.
* @pid: the pid in question.
--- OLD/kernel/workqueue.c~TST 2007-06-02 13:34:57.000000000 +0400
+++ OLD/kernel/workqueue.c 2007-06-02 16:18:02.000000000 +0400
@@ -49,6 +49,7 @@ struct cpu_workqueue_struct {
struct task_struct *thread;

int run_depth; /* Detect run_workqueue() recursion depth */
+ int jobs;
} ____cacheline_aligned;

/*
@@ -253,6 +254,7 @@ static void run_workqueue(struct cpu_wor

cwq->current_work = work;
list_del_init(cwq->worklist.next);
+ cwq->jobs++;
spin_unlock_irq(&cwq->lock);

BUG_ON(get_wq_data(work) != cwq);
@@ -328,7 +330,48 @@ static void insert_wq_barrier(struct cpu
insert_work(cwq, &barr->work, tail);
}

-static int flush_cpu_workqueue(struct cpu_workqueue_struct *cwq)
+extern struct task_struct *get_cpu_curr(int cpu);
+
+static void flush_wait(struct cpu_workqueue_struct *cwq, int cpu, struct completion *done)
+{
+ struct task_struct *curr;
+ struct work_struct *work;
+ int old_pid, jobs;
+
+ if (is_single_threaded(cwq->wq))
+ cpu = raw_smp_processor_id();
+
+again:
+ work = cwq->current_work;
+ jobs = cwq->jobs;
+
+ curr = get_cpu_curr(cpu);
+ old_pid = curr->pid;
+ put_task_struct(curr);
+
+ if (wait_for_completion_timeout(done, HZ * 30))
+ return;
+
+ printk(KERN_ERR "ERR!! %s flush hang: %p %p %d %d\n", cwq->thread->comm,
+ work, cwq->current_work, jobs, cwq->jobs);
+
+ curr = get_cpu_curr(cpu);
+ printk(KERN_ERR "CURR: %d %d %s %ld %ld\n", old_pid, curr->pid,
+ curr->comm, curr->nivcsw, curr->nvcsw);
+ put_task_struct(curr);
+
+ spin_lock_irq(&cwq->lock);
+ list_for_each_entry(work, &cwq->worklist, entry)
+ print_symbol(" %s\n", (unsigned long) work->func);
+ printk(" ----\n");
+ if (cwq->current_work)
+ print_symbol(" %s\n", (unsigned long) cwq->current_work->func);
+ spin_unlock_irq(&cwq->lock);
+
+ goto again;
+}
+
+static int flush_cpu_workqueue(struct cpu_workqueue_struct *cwq, int cpu)
{
int active;

@@ -351,7 +394,7 @@ static int flush_cpu_workqueue(struct cp
spin_unlock_irq(&cwq->lock);

if (active)
- wait_for_completion(&barr.done);
+ flush_wait(cwq, cpu, &barr.done);
}

return active;
@@ -377,7 +420,7 @@ void fastcall flush_workqueue(struct wor

might_sleep();
for_each_cpu_mask(cpu, *cpu_map)
- flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, cpu));
+ flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, cpu), cpu);
}
EXPORT_SYMBOL_GPL(flush_workqueue);

@@ -748,7 +791,7 @@ static void cleanup_workqueue_thread(str
* checks list_empty(), and a "normal" queue_work() can't use
* a dead CPU.
*/
- while (flush_cpu_workqueue(cwq))
+ while (flush_cpu_workqueue(cwq, cpu))
;

kthread_stop(cwq->thread);