KVM

From KVM

This page describes how to count and trace performance events in the KVM kernel module.

There are two tools, kvm_stat and kvm_trace, which were previously used for these tasks. Now this can be done using standard Linux tracing tools.

Contents

Counting events

Often you want event counts after running a benchmark:

$ sudo mount -t debugfs none /sys/kernel/debug
$ sudo ./perf stat -e 'kvm:*' -a sleep 1h
^C
 Performance counter stats for 'sleep 1h':

           8330  kvm:kvm_entry            #      0.000 M/sec
              0  kvm:kvm_hypercall        #      0.000 M/sec
           4060  kvm:kvm_pio              #      0.000 M/sec
              0  kvm:kvm_cpuid            #      0.000 M/sec
           2681  kvm:kvm_apic             #      0.000 M/sec
           8343  kvm:kvm_exit             #      0.000 M/sec
            737  kvm:kvm_inj_virq         #      0.000 M/sec
              0  kvm:kvm_page_fault       #      0.000 M/sec
              0  kvm:kvm_msr              #      0.000 M/sec
            664  kvm:kvm_cr               #      0.000 M/sec
            872  kvm:kvm_pic_set_irq      #      0.000 M/sec
              0  kvm:kvm_apic_ipi         #      0.000 M/sec
            738  kvm:kvm_apic_accept_irq  #      0.000 M/sec
            874  kvm:kvm_set_irq          #      0.000 M/sec
            874  kvm:kvm_ioapic_set_irq   #      0.000 M/sec
              0  kvm:kvm_msi_set_irq      #      0.000 M/sec
            433  kvm:kvm_ack_irq          #      0.000 M/sec
           2685  kvm:kvm_mmio             #      0.000 M/sec

    3.493562100  seconds time elapsed

The perf tool is part of the Linux kernel tree in tools/perf.

Tracing events

Detailed traces can be generated using ftrace:

# mount -t debugfs none /sys/kernel/debug
# echo 1 >/sys/kernel/debug/tracing/events/kvm/enable
# cat /sys/kernel/debug/tracing/trace_pipe
[...]
             kvm-5664  [000] 11906.220178: kvm_entry: vcpu 0
             kvm-5664  [000] 11906.220181: kvm_exit: reason apic_access rip 0xc011518c
             kvm-5664  [000] 11906.220183: kvm_mmio: mmio write len 4 gpa 0xfee000b0 val 0x0
             kvm-5664  [000] 11906.220183: kvm_apic: apic_write APIC_EOI = 0x0
             kvm-5664  [000] 11906.220184: kvm_ack_irq: irqchip IOAPIC pin 11
             kvm-5664  [000] 11906.220185: kvm_entry: vcpu 0
             kvm-5664  [000] 11906.220188: kvm_exit: reason io_instruction rip 0xc01e4473
             kvm-5664  [000] 11906.220188: kvm_pio: pio_read at 0xc13e size 2 count 1
             kvm-5664  [000] 11906.220193: kvm_entry: vcpu 0
^D
# echo 0 >/sys/kernel/debug/tracing/events/kvm/enable

Recording events

Events can be recorded to a file for later reporting and analysis. You can record events for the host using the --host option. You can record events for a guest using the --guest option. You can use both options at the same time to record events for both the host and a guest.

If you use just the --host option the default output file will be perf.data.host. If you use just the --guest option the default output file will be perf.data.guest. If you use both options the default output file will be perf.data.kvm. Use the -o option after the record key word to save the output to a different file name.

# perf kvm --host --guest [kvm options] record -a -o my.perf.data

Recording events for a guest

Using copies of guest files

In order to record events for a guest, the perf tool needs the /proc/kallsyms and /proc/modules for the guest. These are passed to perf with the --guestkallsyms and --guestmodules options. The files will have to be on the host, but you can get them easily using ssh.

# ssh guest "cat /proc/kallsyms" > /tmp/guest.kallsyms
# ssh guest "cat /proc/modules" > /tmp/guest.modules

It is better to use ssh to cat the files and redirect the output than to use scp. Experience has shown that scp guest:/proc/kallsyms /tmp/guest.kallsyms will return an empty file.

Using these options you can record the events for a host and a guest with the following command

# perf kvm --host --guest --guestkallsyms=/tmp/guest.kallsyms --guestmodules=/tmp/guest.modules record -a

The order of the arguments is important. In general, the syntax for using perf to profile kvm is: perf kvm <perf kvm args> <perf command> <perf command args> In this case the perf kvm arguments are --host --guest --guestkallsyms=/tmp/guest.kallsyms --guestmodules=/tmp/guest.modules, the perf command is record, and the perf command argument is -a (profile all processes).

perf will record events until it is terminated with SIGINT. It must be SIGINT. If perf is terminated with any other signal, such as SIGTERM or SIGQUIT, the pref kvm report command (see below) won't correctly process the file generated. It will just list the headers with no data.

Using sshfs

perf kvm has a way of getting the guest's kallsyms and modules by itself instead of you hving to provide them. It makes use of sshfs to mount the root file system of the guest so that it can get the files directly from the guest.

sshfs depends on fuse, the Filesystem in Userspace. If your Linux distribution does not have fuse, see the fuse project page on SourceForge. If your Linux distribution does not have sshfs, see the sshfs project page on SourceForge.

Here's how it works. You create a directory, e.g., /tmp/guestmount. You then create a subdirectory that has for its name the PID of the qemu process for the guest. Then you use sshfs to mount the guest's root file system on that subdirectory. For example:

# mkdir -p /tmp/guestmount
# ps -eo pid,cmd | grep qemu | grep -v grep
24764 /usr/libexec/qemu-kvm -M pc -m 4096 -smp 4 -name guest01 -boot c -drive file=/var/lib/libvirt/images/guest01.img ...
# mkdir /tmp/guestmount/24764
# sshfs -o allow_other,direct_io guest:/ /tmp/guestmount/24764

Then, instead of using the --guestkallsyms and --guestmodules options, you use the --guestmount option.

# perf kvm --host --guest --guestmount=/tmp/guestmount

When you are finished recording, unmount the guest file system using fusermount.

# fusermount -u /tmp/guestmount/24764

Reporting events

Use the perf kvm report command to generate a report from a data file created by perf kvm record. To generate a report for the host, use the --host argument.

# perf kvm --host --guestmount=/tmp/guestmount report

To generate a report for the guest, use the --guest argument.

# perf kvm --guest --guestmount=/tmp/guestmount report

By default the perf report command will read perf.data.host for a host report and perf.data.guest for a guest report. Use the -i option if you want to use a different input file.

# perf kvm --host --guestmount=/tmp/guestmount report -i /tmp/perf.data.kvm

The perf report command sends its output to standard out. If you want the output to go to a file, redirect the output to a file. Don't use the -o option. Contrary to the help text, it does not work.

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