LTP test perf_event_open02.c: possible rounding issue on aarch64 KVM

[Petr Vorel <pvorel@suse.cz>]

Hi,

I have problem with LTP test perf_event_open02.c [1] on QEMU using KVM on
openSUSE aarch64 kernel 5.15.5-1-default (not much different from stable kernel
from kernel.org):

# /opt/ltp/testcases/bin/perf_event_open02
...
perf_event_open02.c:104: TINFO: bench_work estimated loops = 8083 in 500 ms
perf_event_open02.c:149: TINFO: [0] value:2425293761 time_enabled:749092800 time_running:749092800
perf_event_open02.c:149: TINFO: [1] value:2425287027 time_enabled:749141475 time_running:749141475
perf_event_open02.c:149: TINFO: [2] value:2433046583 time_enabled:757346300 time_running:757346300
perf_event_open02.c:149: TINFO: [3] value:2432771537 time_enabled:753369300 time_running:753369300
perf_event_open02.c:149: TINFO: [4] value:2432551620 time_enabled:753784075 time_running:753784075
perf_event_open02.c:149: TINFO: [5] value:2432386104 time_enabled:753481750 time_running:753481750
perf_event_open02.c:149: TINFO: [6] value:2095086137 time_enabled:768866050 time_running:660021525
perf_event_open02.c:308: TINFO: nhw: 6, overall task clock: 4098138525
perf_event_open02.c:309: TINFO: hw sum: 116450294745, task clock sum: 24589636350
perf_event_open02.c:321: TINFO: ratio: 6.000196
perf_event_open02.c:323: TFAIL: test failed (ratio was greater than 6)
...

The test tries to assert the precision of hardware counters (using struct
perf_event_attr hw_event.type = PERF_TYPE_HARDWARE), but sometimes it fails with
slight overrun. We suppose that this is a rounding error, but it'd be nice to
get this confirmed from kernel developers.

Related kernel setup (or you need to know something else)
grep PERF_EVENTS config-5.15.5-1-default # aarch64
CONFIG_HAVE_PERF_EVENTS=y
CONFIG_PERF_EVENTS=y
CONFIG_HW_PERF_EVENTS=y

Test is running inside testing framework with this setup:
qemu-system-aarch64 -device virtio-gpu-pci -only-migratable -chardev ringbuf,id=serial0,logfile=serial0,logappend=on -serial chardev:serial0 -audiodev none,id=snd0 -device intel-hda -device hda-output,audiodev=snd0 -m 2048 -machine virt,gic-version=host -cpu host -mem-prealloc -mem-path /dev/hugepages/ -netdev user,id=qanet0 -device virtio-net,netdev=qanet0,mac=52:54:00:12:34:56 -object rng-random,filename=/dev/urandom,id=rng0 -device virtio-rng-pci,rng=rng0 -boot menu=on,splash-time=5000 -device nec-usb-xhci -device usb-tablet -device usb-kbd -smp 2 -enable-kvm -no-shutdown -vnc :97,share=force-shared -device virtio-serial -chardev pipe,id=virtio_console,path=virtio_console,logfile=virtio_console.log,logappend=on -device virtconsole,chardev=virtio_console,name=org.openqa.console.virtio_console -chardev pipe,id=virtio_console1,path=virtio_console1,logfile=virtio_console1.log,logappend=on -device virtconsole,chardev=virtio_console1,name=org.openqa.console.virtio_console1 -chardev socket,path=qmp_socket,server=on,wait=off,id=qmp_socket,logfile=qmp_socket.log,logappend=on -qmp chardev:qmp_socket -S -device virtio-scsi-pci,id=scsi0 -blockdev driver=file,node-name=hd0-overlay0-file,filename=/var/lib/openqa/pool/7/raid/hd0-overlay0,cache.no-flush=on -blockdev driver=qcow2,node-name=hd0-overlay0,file=hd0-overlay0-file,cache.no-flush=on -device virtio-blk-device,id=hd0-device,drive=hd0-overlay0,bootindex=0,serial=hd0 -blockdev driver=file,node-name=cd0-overlay0-file,filename=/var/lib/openqa/pool/7/raid/cd0-overlay0,cache.no-flush=on -blockdev driver=qcow2,node-name=cd0-overlay0,file=cd0-overlay0-file,cache.no-flush=on -device scsi-cd,id=cd0-device,drive=cd0-overlay0,serial=cd0 -drive id=pflash-code-overlay0,if=pflash,file=/var/lib/openqa/pool/7/raid/pflash-code-overlay0,unit=0,readonly=on -drive id=pflash-vars-overlay0,if=pflash,file=/var/lib/openqa/pool/7/raid/pflash-vars-overlay0,unit=1

Running the same OS and kernel (aarch64 JeOS Tumbleweed 20211202) on RPI it's working:
perf_event_open02.c:104: TINFO: bench_work estimated loops = 3601 in 500 ms
perf_event_open02.c:149: TINFO: [0] value:1080601748 time_enabled:480527015 time_running:480527015
perf_event_open02.c:149: TINFO: [1] value:1080599535 time_enabled:480540573 time_running:480540573
perf_event_open02.c:149: TINFO: [2] value:1080592770 time_enabled:480533868 time_running:480533868
perf_event_open02.c:149: TINFO: [3] value:1080607121 time_enabled:480571573 time_running:480571573
perf_event_open02.c:149: TINFO: [4] value:1080598264 time_enabled:480568330 time_running:480568330
perf_event_open02.c:149: TINFO: [5] value:1080608798 time_enabled:480600001 time_running:480600001
perf_event_open02.c:149: TINFO: [6] value:923390393 time_enabled:480919479 time_running:410947611
perf_event_open02.c:308: TINFO: nhw: 6, overall task clock: 4990107074
perf_event_open02.c:309: TINFO: hw sum: 51868804135, task clock sum: 29940616417
perf_event_open02.c:321: TINFO: ratio: 5.999995
perf_event_open02.c:325: TPASS: test passed

Test is not supported ENOENT when running with similar setup on x86_64 and
s390x quests:
perf_event_open.h:31: TCONF: perf_event_open type/config not supported: ENOENT (2)

grep PERF_EVENTS config-5.15.5-1-default # x86_64
CONFIG_HAVE_PERF_EVENTS=y
CONFIG_PERF_EVENTS=y
CONFIG_PERF_EVENTS_INTEL_UNCORE=y
CONFIG_PERF_EVENTS_INTEL_RAPL=y
CONFIG_PERF_EVENTS_INTEL_CSTATE=y
CONFIG_PERF_EVENTS_AMD_POWER=m
CONFIG_PERF_EVENTS_AMD_UNCORE=m
CONFIG_HAVE_PERF_EVENTS_NMI=y

But it passes on ppc64le

perf_event_open02.c:104: TINFO: bench_work estimated loops = 4075 in 500 ms
perf_event_open02.c:151: TINFO: [0] value:815279669 time_enabled:316461566 time_running:316461566
perf_event_open02.c:151: TINFO: [1] value:815281799 time_enabled:316462740 time_running:316462740
perf_event_open02.c:151: TINFO: [2] value:815280588 time_enabled:316534086 time_running:316534086
perf_event_open02.c:151: TINFO: [3] value:815283285 time_enabled:316465672 time_running:316465672
perf_event_open02.c:151: TINFO: [4] value:815305390 time_enabled:316492698 time_running:316492698
perf_event_open02.c:151: TINFO: [5] value:686550649 time_enabled:316631866 time_running:266632316
perf_event_open02.c:308: TINFO: nhw: 5, overall task clock: 2534004200
perf_event_open02.c:309: TINFO: hw sum: 32612814180, task clock sum: 12669966232
perf_event_open02.c:321: TINFO: ratio: 4.999978
perf_event_open02.c:325: TPASS: test passed

grep PERF_EVENTS config # ppc64le
CONFIG_HAVE_PERF_EVENTS=y
CONFIG_PERF_EVENTS=y
CONFIG_HAVE_PERF_EVENTS_NMI=y

When I tried running aarch64 quest with stable kernel 5.10.76 from kernel.org on
my intel laptop, using simplified setup, the event was not supported (not sure
whether that was caused unavailable -enable-kvm or something else; I also
haven't checked kernel config):

qemu-system-aarch64 -M virt -cpu cortex-a53 -nographic -smp $SMP -kernel Image -append "rootwait root=/dev/vda console=ttyAMA0" -netdev user,id=eth0 -device virtio-net-device,netdev=eth0 -drive file=rootfs.ext4,if=none,format=raw,id=hd0 -device virtio-blk-device,drive=hd0
...
perf_event_open.h:26: TINFO: perf_event_open event.type: 0, event.config: 1
perf_event_open.h:30: TCONF: perf_event_open type/config not supported: ENOENT (2)

I also tested that stable kernel 5.10.76 on RPI but that passed (the same as openSUSE 5.15.5-1-default)
perf_event_open02.c:104: TINFO: bench_work estimated loops = 1496 in 500 ms
perf_event_open02.c:149: TINFO: [0] value:449725668 time_enabled:500191054 time_running:500191054
perf_event_open02.c:149: TINFO: [1] value:449728803 time_enabled:500204795 time_running:500204795
perf_event_open02.c:149: TINFO: [2] value:449732944 time_enabled:500210665 time_running:500210665
perf_event_open02.c:149: TINFO: [3] value:449738099 time_enabled:500210443 time_running:500210443
perf_event_open02.c:149: TINFO: [4] value:449745104 time_enabled:500234961 time_running:500234961
perf_event_open02.c:149: TINFO: [5] value:449756676 time_enabled:500247647 time_running:500247647
perf_event_open02.c:149: TINFO: [6] value:385474224 time_enabled:502975813 time_running:430976612
perf_event_open02.c:308: TINFO: nhw: 6, overall task clock: 4031349522
perf_event_open02.c:309: TINFO: hw sum: 21590362808, task clock sum: 24187113827
perf_event_open02.c:321: TINFO: ratio: 5.999756
perf_event_open02.c:325: TPASS: test passed

So is it a rounding issue on aarch64 QEMU/KVM?
Thanks for any hint what to check / try.

Kind regards,
Petr

[1] https://github.com/linux-test-project/ltp/tree/c2d4836c057fb9f78e7f625d71638d4f40f98659/testcases/kernel/syscalls/perf_event_open/perf_event_open02.c