On Fri, 23 Jul 2021 12:35:09 +0200
Stefan Metzmacher [off-list ref] wrote:

quoted

Assuming this does fix your issue, I sent out a real patch with the
explanation of what happened in the change log, so that you can see why
that change was your issue.  

Yes, it does the trick, thanks very much!

Can I get a "Tested-by" from you?

Now I can finally use:

trace-cmd record -e all -P $(pidof io_uring-cp-forever)

But that doesn't include the iou-wrk-* threads
and the '-c' option seems to only work with forking.

Have you tried it? It should work for threads as well. It hooks to the
sched_process_fork tracepoint, which should be triggered even when a new
thread is created.

Or do you mean that you want that process and all its threads too that are
already running? I could probably have it try to add it via the /proc file
system in that case.

Can you start the task via trace-cmd?

  trace-cmd record -e all -F -c io_uring-cp-forever ...

Is there a way to specify "trace *all* threads of the given pid"?
(Note the threads are comming and going, so it's not possible to
specifiy -P more than once)

Right, although, you could append tasks manually to the set_event_pid file
from another terminal after starting trace-cmd. Once a pid is added to that
file, all children it makes will also be added. That could be a work around
until we have trace-cmd do it.

Care to write a bugzilla report for this feature?

  https://bugzilla.kernel.org/buglist.cgi?component=Trace-cmd%2FKernelshark&list_id=1088173

-- Steve