perfcollect -pid arg results in CPU attribute not set error on Ubuntu

[sywhang]

Originally reported in https://github.com/dotnet/coreclr/issues/18452.

From @paraspatidar:

FYI , apart from kubernetes , same behavior is observed on Ubuntu VM on azure.
However on Ubuntu it happens only when we pass -pid in perfcollect as argument to specifically target perticular dotnet process.
for normal run where it collect for all CPUs (-a) , then it is successfully able to get CPU traces.
(however on kubernetes we have no other option but to use -pid as -a failed due to access issue)
when running that command manually :
/usr/bin/perf script -i perf.data.merged -f comm,pid,tid,cpu,time,event,ip,sym,dso,trace > perf.data.txt :
output : Samples for 'cpu-clock' event do not have CPU attribute set
OS :
NAME="Ubuntu"
VERSION="18.04.1 LTS (Bionic Beaver)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.04.1 LTS"
VERSION_ID="18.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic
uname -r : 4.18.0-1011-azure

[lathiat]

I found this issue on Google when researching a similar error but in a different software suite.. sharing a few notes here in case someone comes back to this ticket:

I believe the reason this happens is that if you are on metal with hardware performance counters, perf uses the 'cycles' event that has the CPU number. If you are on a VM without hardware performance counters (which is most of the time, particularly in public clouds due to security reasons, I think some AWS instance types where you have dedicated hardware access might give you some performance counters even in a VM), then it uses the software 'cpu-clock' event which does not have this field.

I didn't get to figuring out why in some configurations using all-CPU versus per-process mode seems to result in the CPU field appearing or disappearing.

When using "perf script"'s "-F" field specifier you can specify different fields for "software", "hardware" and "trace" events. Unfortunately if you specify a format for hardware events and there are no such in the file then it errors out. So it seems we can't ask "perf script" to handle this for us and likely some kind of software detection is going to be required.

Hope it helps someone...