Negative stack id and pid error when profiling ustack event

[12ff54e]

Hi,

My linux server has an unusual high cpu usage and htop shows nothing. So I first check the kernel stack using bpftrace -e 'profile:hz:99 /cpu == 0/ { @[kstack] = count(); }' but most of the time it's empty. When I profile user-level stack it reported an error

ERROR: failed to look up stack id -17 (pid -1): -1

then prints out lots of stack info with addresses only. What does this error message mean? I try google it but no luck.

[viktormalik]

This error appears when bpftrace cannot find an entry in its internal map for keeping stack traces. What's interesting here is the error code, -17 is -EEXISTS. The only place where EEXISTS is returned from bpf_map_lookup_elem is when BPF_F_REUSE_STACKID is set but we should never set that flag (at least per this comment).

Would be nice if we could reproduce this to investigate more (the above command works well on my localhost).

[12ff54e]

Thanks for your swift reply! I installed bpftrace using apt and the version number is v0.14.0 (maybe a bit old). I will try with the latest verion to check if this error persist on my server.

[viktormalik]

Yes, that is certainly the first thing to try.

[12ff54e]

The lasteat version works without that error message. However the user-level stack info is still consist of address only, any suggestion on how do I can get more infomation out of it? This is my 1st time using bpftrace and I'm completely a noob about it.

Thanks