Accuracy of rte_get_tsc_hz() compared to linux

[Isaac Boukris]

On Thu, Sep 19, 2024 at 2:27 AM Stephen Hemminger
<stephen at networkplumber.org> wrote:
>
> On Thu, 19 Sep 2024 01:04:40 +0300
> Isaac Boukris <iboukris at gmail.com> wrote:
>
> > I've run the helloworld application on an isolated cpu:
> > taskset -c 10 ./dpdk-helloworld --log-level=lib.eal:debug --no-huge
> >
> > The results are:
> > EAL: TSC frequency arch ~2100000 KHz
> > EAL: TSC frequency linux ~2095082 KHz
> > EAL: TSC frequency estimate ~2095346 KHz
> >
> > The arch one is picked, which seems rather wrong, any way to override that?
> > Should we lower the estimation rounding to 1MHz or even 1KHz in the linux one?
> >
> > Thoughts? Thanks!
> >
> > Kernel: 4.18.0-513.9.1.el8_9.x86_64
>
> Note: 4.18 kernel was end of life 12 August 2018, I assume
> this is RHEL8 which does their own backports and never changes kernel version.

Indeed RHEL 8.9

> What is the kernel dmesg, why is it deciding on that value?

It comes from kernel's determine_cpu_tsc_frequencies() afaict (which
didn't change that much).

As a matter of fact, I got a similar behavior on my vmware VM on my
laptop (although smaller diff).

kernel: 4.18.0-553.8.1.el8_10.x86_64

lscpu:
Model name:          12th Gen Intel(R) Core(TM) i7-1260P
Stepping:            3
CPU MHz:             2495.994
BogoMIPS:            4991.98
Hypervisor vendor:   VMware

dmesg | grep -i tsc
[    0.000000] vmware: TSC freq read from hypervisor : 2495.994 MHz
[    0.000000] tsc: Detected 2495.994 MHz processor
[    0.000000] TSC deadline timer available
[    0.010000] clocksource: tsc-early: mask: 0xffffffffffffffff
max_cycles: 0x23fa717cb36, max_idle_ns: 440795237972 ns
[    0.905000] clocksource: Switched to clocksource tsc-early
[    3.104381] tsc: Refined TSC clocksource calibration: 2495.990 MHz
[    3.105473] clocksource: tsc: mask: 0xffffffffffffffff max_cycles:
0x23fa6db1dfc, max_idle_ns: 440795265852 ns
[    3.264297] clocksource: Switched to clocksource tsc

cat /sys/devices/system/cpu/cpu0/tsc_freq_khz
2495990

sudo bpftrace -e 'BEGIN { printf("%u\n", *kaddr("tsc_khz")); exit(); }'
Attaching 1 probe...
2495990

My modified dpdk code above gives:
EAL: TSC frequency arch ~0 KHz
EAL: TSC frequency linux ~2495982 KHz
EAL: TSC frequency estimate ~2497263 KHz

Note that with the unmodified dpdk code which rounds to 10MHz both the
linux and the common estimation would give 2500000 KHz.