Hi Steven,

On Wed, Jul 15, 2020 at 10:36 PM Steven Rostedt [off-list ref] wrote:

To rule out that this may be an issue with the function_graph tracer,
can you also try it with:

 ftrace=function ftrace_filter=worqueue_prepare_cpu

And see if the function tracer shows anything.

/me just looked at the code.

Yes, the function tracer is set up to trace really early, but the
function_graph tracer is initialized by core_initcall(), which is much
later.

Perhaps try this patch (note, it's from latest mainline):

And retry the function_graph tracer.

With this patch, the function graph tracer doesn't work at boot time.
Please see the log below:

# mount -t debugfs debugfs /sys/kernel/debug/
# cd /sys/kernel/debug/tracing/trace
# cat current_tracer
nop

Note that I tested it both on 5.4.47-rt28 and 5.6.19-rt11 (but not on
latest mainline).

Regarding the function tracer, you were correct: it can trace
workqueue_prepare_cpu
without any issue.

Thank you,
Itai Handler