lkml.org 
[lkml]   [2013]   [Jun]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: stop_machine lockup issue in 3.9.y.
On 06/05/2013 02:11 PM, 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.

Just to be clear on the ath9k part for the wifi folks:

This is basically un-patched 3.9.4, but I have 200 virtual stations
configured on each of two ath9k radios. I cannot reproduce the problem
without ath9k, but I do not know for certain ath9k is the real
culprit.

In the case where I can most easily reproduce the lockup, ath9k virtual
stations would be trying to associate, so I'd expect a fair amount
of packet processing to be happening...

> 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?

Thanks,
Ben



--
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc http://www.candelatech.com



\
 
 \ /
  Last update: 2013-06-06 01:21    [W:0.682 / U:0.032 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site