Re: [RFC][PATCH] tracing: Allow creating instances with specified system events

[Dmytro Maluka <dmaluka@chromium.org>]

On Mon, Nov 27, 2023 at 05:41:08PM -0500, Steven Rostedt wrote:
> From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
> 
> A trace instance may only need to enable specific events. As the eventfs
> directory of an instance currently creates all events which adds overhead,
> allow internal instances to be created with just the events in systems
> that they care about. This currently only deals with systems and not
> individual events, but this should bring down the overhead of creating
> instances for specific use cases quite bit.
> 
> The qla2xxx instance could just enable the systems it cares about, but that
> should be a separate patch.
> 
> Note that kprobes and synthetic events created after the creation of these
> instances, will be added to these instances, but those that are created
> before the creation of the instance will not be included.

This limitation will cause (unrelated) events created by modules that
are insmoded after creating the instance to be also added to the
instance. Why not filter those as well?

Besides that, the change looks nice to me.

> Note, this may also be useful for creating instances in the eventfs, but
> I'm not sure how to do this there. I could add a deliminator:
> 
>   mkdir /sys/kernel/tracing/instances/foo::sched,timer
> 
> But if a tool already uses "::" as a deliminator, then this will break it.
> I could just have it work if all the events after the deliminator exist.
> 
>   Thoughts?
> 
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
>  drivers/scsi/qla2xxx/qla_os.c       |  2 +-
>  include/linux/trace.h               |  4 ++--
>  kernel/trace/trace.c                | 22 ++++++++++++--------
>  kernel/trace/trace.h                |  3 ++-
>  kernel/trace/trace_boot.c           |  2 +-
>  kernel/trace/trace_events.c         | 31 ++++++++++++++++++++++++++---
>  samples/ftrace/sample-trace-array.c |  2 +-
>  7 files changed, 49 insertions(+), 17 deletions(-)