Help with trace-cmd/ftrace recording process ID information

From: Will Hawkins
Date: Mon Jul 17 2017 - 15:18:25 EST



Hello everyone, especially Mr. Rostedt,

I have had great success with ftrace debugging performance issues on Linux systems. The combination of ftrace and trace-cmd are absolutely amazing tools for digging in to exactly what is going on in a system and where performance problems exist.

I recently switched to a different host and attempted to run trace-cmd record to get a record of page faults:

/path/to//trace-cmd/trace-cmd record -e page_fault_user /bin/ls

When I "report" on that trace, I get entries like the following:

<...>-41850 [010] 27484983.185200: page_fault_user: address=__per_cpu_end ip=__per_cpu_end error_code=0x14

It's exactly what I want. However, it does not list the process that generated that fault. Instead, it uses <...>. I dug into the trace-cmd code and see where this is generated and why it is generated.

What I don't understand is why on a different system, when I run the same record command, I get the following output:

ls-19887 [005] 2438162.263793: page_fault_user: address=__per_cpu_end ip=__per_cpu_end error_code=0x14

Again, it's exactly what I want and it lists the process name that generated the fault.

From the code, I see that the <...> is printed instead of the name of the process when the pid is not in the pevent's command lines. What I can't seem to figure out is why the process would be in that list on one host and not on the other.

When I looked at the trace.dat file directly, I did notice that on the "good" host, there are a list of pids and names. On the "bad" host, there is no such list in the trace.dat file. I am sure that is the reason for the <...>s being printed, but I can't figure out why that list is not getting in the trace.dat file.

I gave a quick look to try to find where that pid/comm list is generated and written to the trace.dat file, but couldn't find anything.

I figured that I would send an email before I dug any further in case someone has seen this already. I am happy to pass along other pertinent information if it is helpful to debug the problem. I just don't want to spam the list with information that is irrelevant.

Again, the combination of ftrace/trace-cmd is borderline magic. I appreciate all the work that has gone into it!

Thanks in advance for helping me sort through this issue!

Will