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. -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
