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

From: Purcareata Bogdan
Date: Mon Feb 23 2015 - 02:29:44 EST

On 20.02.2015 16:54, Sebastian Andrzej Siewior wrote:
On 02/20/2015 03:12 PM, Paolo Bonzini wrote:
Thomas, what is the usual approach for patches like this? Do you take
them into your rt tree or should they get integrated to upstream?

Patch 1 is definitely suitable for upstream, that's the reason why we
have raw_spin_lock vs. raw_spin_unlock.

raw_spin_lock were introduced in c2f21ce2e31286a0a32 ("locking:
Implement new raw_spinlock). They are used in context which runs with
IRQs off - especially on -RT. This includes usually interrupt
controllers and related core-code pieces.

Usually you see "scheduling while atomic" on -RT and convert them to
raw locks if it is appropriate.

Bogdan wrote in 2/2 that he needs to limit the number of CPUs in oder
not cause a DoS and large latencies in the host. I haven't seen an
answer to my why question. Because if the conversation leads to
large latencies in the host then it does not look right.

What I did notice were bad cyclictest results, when run in a guest with 24 VCPUs. There were 24 netperf flows running in the guest. The max cyclictest latencies got up to 15ms in the guest, however I haven't captured any host side information related to preemptirqs off statistics.

What I was planning to do in the past days was to rerun the test and come up with the host preemptirqs off disabled statistics (mainly the max latency), so I could have a more reliable argument. I haven't had the time nor the setup to do that yet, and will come back with this as soon as I have them available.

Each host PIC has a rawlock and does mostly just mask/unmask and the
raw lock makes sure the value written is not mixed up due to
This hardly increase latencies because the "locked" path is very short.
If this conversation leads to higher latencies then the locked path is
too long and hardly suitable to become a rawlock.

From my understanding, the kvm openpic emulation code does more than just that - it requires to be atomic with interrupt delivery. This might mean the bad cyclictest max latencies visible from the guest side (15ms), may also have a correspondent to how much time that raw spinlock is taken, leading to an unresponsive host.

Best regards,
Bogdan P.



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