RCU stall with high number of KVM vcpus

[Marc Zyngier]

On 13/11/17 18:40, Jan Glauber wrote:
> On Mon, Nov 13, 2017 at 06:11:19PM +0000, Marc Zyngier wrote:
>> On 13/11/17 17:35, Jan Glauber wrote:
>>> On Mon, Nov 13, 2017 at 01:47:38PM +0000, Marc Zyngier wrote:
> 
> [...]
> 
>>>> Please elaborate. Messed in what way? Corrupted? The guest crashing? Or
>>>> is that a tooling issue?
>>>
>>> Every vcpu that oopses prints one line in parallel, so I get blocks like:
>>> [58880.179814] [<ffff000008084b98>] ret_from_fork+0x10/0x18
>>> [58880.179834] [<ffff000008084b98>] ret_from_fork+0x10/0x18
>>> [58880.179847] [<ffff000008084b98>] ret_from_fork+0x10/0x18
>>> [58880.179873] [<ffff000008084b98>] ret_from_fork+0x10/0x18
>>> [58880.179893] [<ffff000008084b98>] ret_from_fork+0x10/0x18
>>> [58880.179911] [<ffff000008084b98>] ret_from_fork+0x10/0x18
>>> [58880.179917] [<ffff000008084b98>] ret_from_fork+0x10/0x18
>>> [58880.180288] [<ffff000008084b98>] ret_from_fork+0x10/0x18
>>> [58880.180303] [<ffff000008084b98>] ret_from_fork+0x10/0x18
>>> [58880.180336] [<ffff000008084b98>] ret_from_fork+0x10/0x18
>>> [58880.180363] [<ffff000008084b98>] ret_from_fork+0x10/0x18
>>> [58880.180384] [<ffff000008084b98>] ret_from_fork+0x10/0x18
>>> [58880.180415] [<ffff000008084b98>] ret_from_fork+0x10/0x18
>>> [58880.180461] [<ffff000008084b98>] ret_from_fork+0x10/0x18
>>>
>>> I can send the full log if you want to have a look.
>>
>> Sure, send that over (maybe not over email though).
> 
> Here is the guest dmesg:
> http://paste.ubuntu.com/25955682/

Yeah, that's because all the vcpus are getting starved at the same time,
and spitting out interleaved traces... Not very useful anyway, as I
think this is only a consequence of what's happening on the host.

> 
> And the host dmesg as it might have been too big for the lists:
> http://paste.ubuntu.com/25955699/

And that one doesn't show much either, apart from indicating that
something is keeping the lock for itself. Drat.

We need to narrow down the problem, or make it appear on more common HW.
Let me know if you've managed to reproduce it with non-VHE and/or on TX-1.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...