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

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Date:	Fri, 20 Feb 2015 16:06:06 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	Alexander Graf <agraf@...e.de>,
	Bogdan Purcareata <bogdan.purcareata@...escale.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-rt-users@...r.kernel.org
CC:	linux-kernel@...r.kernel.org, scottwood@...escale.com,
	mihai.caraman@...escale.com, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/2] powerpc/kvm: Enable running guests on RT Linux

On 02/20/2015 03:57 PM, Paolo Bonzini wrote:
> 
> 
> On 20/02/2015 15:54, Sebastian Andrzej Siewior wrote:
>> 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.
>>
>> 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
>> preemption.
>> 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.
> 
> 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.
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?

> Paolo
> 

Sebastian
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