[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] [PATCH] RFC: Linux: disable APERF/MPERF feature in PV kernels

On 05/22/2012 07:18 PM, Konrad Rzeszutek Wilk wrote:
On Tue, May 22, 2012 at 06:07:11PM +0200, Andre Przywara wrote:

while testing some APERF/MPERF semantics I discovered that this
feature is enabled in Xen Dom0, but is not reliable.
The Linux kernel's scheduler uses this feature if it sees the CPUID
bit, leading to costly RDMSR traps (a few 100,000s during a kernel
compile) and bogus values due to VCPU migration during the

Can you point me to the Linux scheduler code that does this? Thanks.

arch/x86/kernel/cpu/sched.c contains code to read out and compute APERF/MPERF registers. I added a Xen debug-key to dump a usage counter added in traps.c and thus could prove that it is actually the kernel that accesses these registers. As far as I understood this the idea is to learn about boosting and down-clocking (P-states) to get a fairer view on the actual computing time a process consumed.

The attached patch explicitly disables this CPU capability inside
the Linux kernel, I couldn't measure any APERF/MPERF reads anymore
with the patch applied.
I am not sure if the PVOPS code is the right place to fix this, we
could as well do it in the HV's xen/arch/x86/traps.c:pv_cpuid().
Also when the Dom0 VCPUs are pinned, we could allow this, but I am
not sure if it's worth to do so.

Awaiting your comments.


P.S. Of course this doesn't fix pure userland software like
cpupower, but I would consider this in the user's responsibility to

Which would not work anymore as the cpufreq support is disabled
when it boots under Xen.

Do you mean with "anymore" in a future kernel? I tested this on 3.4.0 and cpupower monitor worked fine. Right, cpufreq is not enabled, but cpupower uses the /dev/cpu/<n>/msr device file to directly read the MSRs. So I get this output if run on an idle Dom0:

CPU | C0   | Cx   | Freq
   0|  0.10| 99.90|  3473


not use these tools in Dom0, but instead use xenpm.

Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany

commit e802e47d85314b4541288e4a19d057e2ea885a28
Author: Andre Przywara<andre.przywara@xxxxxxx>
Date:   Tue May 22 15:13:07 2012 +0200

     filter APERFMPERF feature in Xen to avoid kernel internal usage

     Signed-off-by: Andre Przywara<andre.przywara@xxxxxxx>

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 95dccce..71252d5 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -240,6 +240,11 @@ static void xen_cpuid(unsigned int *ax, unsigned int *bx,
                *dx = cpuid_leaf5_edx_val;

+       case 6:
+               /* Disabling APERFMPERF for kernel usage */
+               maskecx = ~(1U<<  0);
+               break;
        case 0xb:
                /* Suppress extended topology stuff */
                maskebx = 0;

Xen-devel mailing list

Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 448-3567-12

Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.