Greetings,

When tracing via sysFS and keeping the default configuration,
everything that is happening on a processor is logged. That is called
"CPU-wide". Any process that get scheduled out of the processor won't
be traced. On perf one can execute:
# perf record -e cs_etm/@20070000.etr/ --per-thread my_application (example 1)
# perf record -e cs_etm/@20070000.etr/ -C 2,3,4 my_application (example 2)
For example 1, perf will switch on the tracer associated to the CPU
where my_application has been installed for execution. If the process
gets scheduled on a different CPU perf will do the right thing and
follow it around. That is called "per-thread".
In example 2 everything that is happening on CPU 2,3,4 will be traced
for as long as my_application is executing, regardless of where
my_application is executing. That is also a CPU-wide trace scenario.

I was more under the impression that CPU-wide records everything that the CPU executes

(this is achieved by using sysfs like you described), regardless of the term "thread".

I don't really understand why CPU-wide is the right term for example 2. Both of the examples

record "per-thread", except of the CPU mask. Example 1 doesn't mask any CPUs, where

example 2 masks all CPUs except 2, 3, 4, It still doesn't record "CPU-wide", only "per-thread",

but on non-masked CPUs(if it weren't "per-thread", it wouldn't care about scheduling a thread and disable/enable accordingly).

I'm a little confused, It really seems like sysfs==cpu-wide and perf==per-thread. Perhaps chagning

the modes to "CS_MODE_PER_THREAD", "CS_MODE_CPU_WIDE" and make the sysfs and perf implementation

use these modes is something that solves the puzzle for me.

As such you will find places like that where things aren't exactly how

they should be - heck, I find them in my original code all the time.
You should test it but once again I think you are correct -
coresight_enable_source() should be called from etm_event_start().

After a second look, actually i think calling coresight_enable_source() will be problematic.

Using the sysfs implementation(coresight.c) from perf is problematic, since it maintains a reference count

per-device. If there's a sysfs session running on a tracer, and perf uses the __same__ path to the

sink and the same source, using coresight_enable_path() will result in simply increasing the reference count without returning any errors..

So calling source_ops(source)->enable() directly actually will result in an error, which is the expected behavior(?)

Also, just out of curiosity, what happens when perf is requested to record a multithreaded process? the "per-thread" mechanism doesn't seem to fit here, because of the "single tracer to single-sink" rule.