linux-kernel - Re: [PATCH] synth_events: Do not block other dyn

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Message-ID: <20211001155746.GA16348@kbox>
Date:   Fri, 1 Oct 2021 08:57:46 -0700
From:   Beau Belgrave <beaub@...ux.microsoft.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     linux-trace-devel@...R.KERNEL.ORG, rostedt@...dmis.org,
        zanussi@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] synth_events: Do not block other dyn_event systems
 during create

> Thanks for clean up the synthetic event!
> This looks good to me.
> 
> Reviewed-by: Masami Hiramatsu <mhiramat@...nel.org>
No problem, thanks for the review!

> So now are you trying to reuse synth event for user event?
> Then I think you need to register a new dyn_event ops so
> that histogram will not submit the event.
A user events patch will get sent out soon, I'm finalizing some testing.
User events will register it's own dyn_event ops and allow users the
option to create, delete and view status of user events via the
dynamic_events tracefs file (in addition to the user mode IOCTL/ABI).

As probes attach to the user events the status of this is reflected in
dynamic_events, which makes it easy for admins to see why one is busy,
etc. It also makes it easy to verifying the system is working as
expected with just a terminal.

> BTW, how do you filter an event written by a user process?
> Will you add an array of event id for the file data structure?
The filtering would happen at the trace_event level, the only filtering
at the user event level is if a probe has been enabled on the underlying
trace_event or not. That is done via the shared page bits being cleared
or set. Bits are updated as probe un/registrations occur.

Users can advertise field values, offsets, etc as well as the print_fmt
in the newer ABI/patch based on the feedback from LPC2021. These land in
the trace_event fields and are viewable via tracefs like any other
evnet. I hope that is enough to light up the history feature of trace_event.
I am not familiar with it as much as you all.

Thanks,
-Beau