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

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Date:	Thu, 12 Jul 2012 07:29:23 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	David Ahern <dsahern@...il.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 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.

> 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.

> I won't have much time over the next few days to run much in the way
> of tests; I'll come back to it Sunday night.
> 
> David
> 
> >>
> >>You also need to clear TR, BTS, BTINT from MSR_IA32_DEBUGCTLMSR and
> >>ideally you'd also clear MSR_IA32_DS_AREA so that any write will be a
> >>proper NULL deref or such.
> >Yes. With the patch above :pp modifier does not crash guest for me, but
> >in theory it should since BTS are still written to DS. May be BTS writes do
> >not overshoot guest entry. Will have to ask Intel for clarification.

--
			Gleb.
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