Re: sched: spinlock recursion in sched_rr_get_interval

[Sasha Levin <sasha.levin@oracle.com>]

On 12/28/2014 03:17 PM, Davidlohr Bueso wrote:
> On Sat, 2014-12-27 at 10:52 -0500, Sasha Levin wrote:
>> > There's a chance that lock->owner would change, but how would you explain
>> > it changing to 'current'?
> So yeah, the above only deals with the weird printk values, not the
> actual issue that triggers the BUG_ON. Lets sort this out first and at
> least get correct data.

Is there an issue with weird printk values? I haven't seen a report of
something like that, nor have seen it myself.

>> > That is, what race condition specifically creates the
>> > 'lock->owner == current' situation in the debug check?
> Why do you suspect a race as opposed to a legitimate recursion issue?
> Although after staring at the code for a while, I cannot see foul play
> in sched_rr_get_interval.
> 
> Given that all reports show bogus contending CPU and .owner_cpu, I do
> wonder if this is actually a symptom of the BUG_ON where something fishy
> is going on.. although I have no evidence to support that. I also ran
> into this https://lkml.org/lkml/2014/11/7/762 which shows the same bogus
> values yet a totally different stack.
> 
> Sasha, I ran trinity with CONFIG_DEBUG_SPINLOCK=y all night without
> triggering anything. How are you hitting this?

I don't have any reliable way of reproducing it. The only two things I
can think of are:

 - Try running as root in a disposable vm
 - Try running with really high load (I use ~800 children on 16 vcpu
guests).


Thanks,
Sasha