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    SubjectRe: perf with precise attribute kills all KVM based VMs
    On Thu, Jul 12, 2012 at 09:20:53AM -0600, David Ahern wrote:
    > 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).
    cycles:ppG and cycles:pG should be illegal. Peter's patch takes care of
    this. Others should set exclude guest and have to work without blowing
    up VMs.

    >
    > 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
    Do not run perf kvm. It does not set exclude_guest and :p and :pp is not
    compatible with guest profiling and should be disallowed. Again Peter's
    patch takes care of this.

    Run perf top -e cycles:pH or similar.

    >
    > Note the :pH this time.
    I am not sure what perf kvm does with :pH modifier, but H modifier does
    not make sense with perf kvm and should be reported as an error by perf tool.

    >
    > 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().
    Isn't this a bug? Why anything at all resets exclude-host set by
    perf-kvm?


    >
    > So, is the idea of your patch to not enable the PEBS in guest mode?
    >
    Yes, the idea of my patch is to disable PEBS and the guest entry.

    --
    Gleb.


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