linux-kernel - Re: perf with precise attribute kills all KVM based VMs

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Date:	Thu, 12 Jul 2012 09:20:53 -0600
From:	David Ahern <dsahern@...il.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Peter Zijlstra <peterz@...radead.org>, Avi Kivity <avi@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: perf with precise attribute kills all KVM based VMs

On 7/11/12 10:29 PM, Gleb Natapov wrote:
> On Wed, Jul 11, 2012 at 10:11:57PM -0600, David Ahern wrote:
>> On 7/11/12 3:53 AM, Gleb Natapov wrote:
>>> On Wed, Jul 11, 2012 at 11:49:47AM +0200, Peter Zijlstra wrote:
>>>> On Wed, 2012-07-11 at 10:10 +0300, Gleb Natapov wrote:
>>>>
>>>>> Looks like Avi is right about the overshoot. Can you test something like this?
>>>>>
>>>>> diff --git a/arch/x86/kernel/cpu/perf_event_intel.c b/arch/x86/kernel/cpu/perf_event_intel.c
>>>>> index 166546e..5fb371a 100644
>>>>> --- a/arch/x86/kernel/cpu/perf_event_intel.c
>>>>> +++ b/arch/x86/kernel/cpu/perf_event_intel.c
>>>>> @@ -1374,8 +1374,11 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr)
>>>>>   	arr[0].msr = MSR_CORE_PERF_GLOBAL_CTRL;
>>>>>   	arr[0].host = x86_pmu.intel_ctrl & ~cpuc->intel_ctrl_guest_mask;
>>>>>   	arr[0].guest = x86_pmu.intel_ctrl & ~cpuc->intel_ctrl_host_mask;
>>>>> +	arr[1].msr = MSR_IA32_PEBS_ENABLE;
>>>>> +	arr[1].host = cpuc->pebs_enabled;
>>>>> +	arr[1].guest = 0;
>>>>> +	*nr = 2;
>>>>>
>>>>> -	*nr = 1;
>>>>>   	return arr;
>>>>>   }
>>>>
>>
>> So far the 64-bit Fedora 10 VM with both a Fedora 10 stock kernel
>> and a 2.6.38 kernel have not faired well - and that's the only VM I
>> have tried at the moment. Using -e cycles:pp I have been able to
>> lock up the VM 3 times out of 3 series of tests with perf-kvm that
>> includes network traffic (e.g., netperf), disk I/O (dd based to
>> create a file with dsync flag) and pure userspace cpu bound (openssl
>> speed). May or may not be related.
>>
> OK that's may be BTSes. What about -e cycles:p? BTW are you using your
> patch to set exclude_guest parameter? If not use -e cycles:Hp.

I started with cycles:pp; should not really matter - they all need to 
work without blowing up VMs (cycles:p, cycles:pH, cycles:pG, cycles:pp, 
cycles:ppH, cycles:ppG).

For grins I ran a quick test while reading emails this morning. This 
time a fedora 17 VM with 3.4.0-1.fc17.x86_64 kernel. It too locks up 
pretty quickly - just a couple of iterations of perf:

perf kvm --guestmount=/tmp/guest-mount record -fo /tmp/perf.data -a -v 
-e cycles:pH -- sleep 60

Note the :pH this time.

I did not have netserver installed in the VM so used a ping flood for 
network traffic.

>
>> Also, I noted that 'perf kvm --guest record -e cycles:pp' does not
>> generate a whole lot of samples -- like < 100 in a 20-second sample
>> -- despite the fact that the guest is rather busy.
>>
> Host events do not suppose to generate events while guest is running.

My server has 16 cpus and the VM has only 2 vcpus; with the -a I would 
expect some host sampling. Note: in the above case :pp resets the 
exclude-host modifier set by the perf-kvm part, so hosts samples are not 
excluded. See parse_events_modifier().

So, is the idea of your patch to not enable the PEBS in guest mode?

David
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