Re: stop_machine lockup issue in 3.9.y.

From: Eric Dumazet
Date: Wed Jun 05 2013 - 21:35:08 EST


On Wed, 2013-06-05 at 14:11 -0700, Tejun Heo wrote:
> (cc'ing wireless crowd, tglx and Ingo. The original thread is at
> http://thread.gmane.org/gmane.linux.kernel/1500158/focus=55005 )
>
> Hello, Ben.
>
> On Wed, Jun 05, 2013 at 01:58:31PM -0700, Ben Greear wrote:
> > Hmm, wonder if I found it. I previously saw times where it appears
> > jiffies does not increment. __do_softirq has a break-out based on
> > jiffies timeout. Maybe that is failing to get us out of __do_softirq
> > in my lockup case because for whatever reason the system cannot update
> > jiffies in this case?
> >
> > I added this (probably whitespace damaged) hack and now I have not been
> > able to reproduce the problem.
>
> Ah, nice catch. :)
>
> > diff --git a/kernel/softirq.c b/kernel/softirq.c
> > index 14d7758..621ea3b 100644
> > --- a/kernel/softirq.c
> > +++ b/kernel/softirq.c
> > @@ -212,6 +212,7 @@ asmlinkage void __do_softirq(void)
> > unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
> > int cpu;
> > unsigned long old_flags = current->flags;
> > + unsigned long loops = 0;
> >
> > /*
> > * Mask out PF_MEMALLOC s current task context is borrowed for the
> > @@ -241,6 +242,7 @@ restart:
> > unsigned int vec_nr = h - softirq_vec;
> > int prev_count = preempt_count();
> >
> > + loops++;
> > kstat_incr_softirqs_this_cpu(vec_nr);
> >
> > trace_softirq_entry(vec_nr);
> > @@ -265,7 +267,7 @@ restart:
> >
> > pending = local_softirq_pending();
> > if (pending) {
> > - if (time_before(jiffies, end) && !need_resched())
> > + if (time_before(jiffies, end) && !need_resched() && (loops < 500))
> > goto restart;
>
> So, softirq most likely kicked off from ath9k is rescheduling itself
> to the extent where it ends up locking out the CPU completely. The
> problem is usually okay because the processing would break out in 2ms
> but as jiffies is stopped in this case with all other CPUs trapped in
> stop_machine, the loop never breaks and the machine hangs. While
> adding the counter limit probably isn't a bad idea, softirq requeueing
> itself indefinitely sounds pretty buggy.
>
> ath9k people, do you guys have any idea what's going on? Why would
> softirq repeat itself indefinitely?
>
> Ingo, Thomas, we're seeing a stop_machine hanging because
>
> * All other CPUs entered IRQ disabled stage. Jiffies is not being
> updated.
>
> * The last CPU get caught up executing softirq indefinitely. As
> jiffies doesn't get updated, it never breaks out of softirq
> handling. This is a deadlock. This CPU won't break out of softirq
> handling unless jiffies is updated and other CPUs can't do anything
> until this CPU enters the same stop_machine stage.
>
> Ben found out that breaking out of softirq handling after certain
> number of repetitions makes the issue go away, which isn't a proper
> fix but we might want anyway. What do you guys think?
>

Interesting....

Before 3.9 and commit c10d73671ad30f5469
("softirq: reduce latencies") we used to limit the __do_softirq() loop
to 10.



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