Re: [PATCH 0/2] powerpc/kvm: Enable running guests on RT Linux

From: Purcareata Bogdan
Date: Mon Feb 23 2015 - 03:13:09 EST

On 20.02.2015 17:17, Sebastian Andrzej Siewior wrote:
On 02/20/2015 04:10 PM, Paolo Bonzini wrote:
On 20/02/2015 16:06, Sebastian Andrzej Siewior wrote:
On 02/20/2015 03:57 PM, Paolo Bonzini wrote:

Yes, but large latencies just mean the code has to be rewritten (x86
doesn't anymore do event injection in an atomic regions for example).
Until it is, using raw_spin_lock is correct.

It does not sound like it. It sounds more like disabling interrupts to
get things run faster and then limit it on a different corner to not
blow up everything.

"This patchset enables running KVM SMP guests with external interrupts
on an underlying RT-enabled Linux. Previous to this patch, a guest with
in-kernel MPIC emulation could easily panic the kernel due to preemption
when delivering IPIs and external interrupts, because of the openpic
spinlock becoming a sleeping mutex on PREEMPT_RT_FULL Linux".

Max latencies was decreased "Max latency (us) 70 62" and that
is why this is done? For 8 us and possible DoS in case there are too
many cpus?

My understanding is that:

1) netperf can get you a BUG KVM, and raw_spinlock fixes that

Actually, it's not just netperf. The bug triggers in the following scenarios:
- running CPU intensive task (while true; do date; done) in SMP guest (even with 2 VCPUs)
- running netperf in guest
- running cyclictest in SMP guest

May I please see a backtrace with context tracking which states where
the interrupts / preemption gets disabled and where the lock was taken?

Will do, I will get back to you as soon as I have it available. I will try and capture it using function trace.

I'm not totally against this patch I just want to make sure this is not
a blind raw conversation to shup up the warning the kernel throws.

2) cyclictest did not trigger the BUG, and you can also get reduced
latency from using raw_spinlock.

I think we agree that (2) is not a factor in accepting the patch.
good :)



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