linux-kernel - Re: [PATCH 2/4] tracing/user_events: Introduce multi-format events

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Date: Mon, 29 Jan 2024 09:29:07 -0800
From: Beau Belgrave <beaub@...ux.microsoft.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com
Subject: Re: [PATCH 2/4] tracing/user_events: Introduce multi-format events

On Fri, Jan 26, 2024 at 03:04:45PM -0500, Steven Rostedt wrote:
> On Fri, 26 Jan 2024 11:10:07 -0800
> Beau Belgrave <beaub@...ux.microsoft.com> wrote:
> 
> > > OK, so the each different event has suffixed name. But this will
> > > introduce non C-variable name.
> > > 
> > > Steve, do you think your library can handle these symbols? It will
> > > be something like "event:[1]" as the event name.
> > > Personally I like "event.1" style. (of course we need to ensure the
> > > user given event name is NOT including such suffix numbers)
> > >   
> > 
> > Just to clarify around events including a suffix number. This is why
> > multi-events use "user_events_multi" system name and the single-events
> > using just "user_events".
> > 
> > Even if a user program did include a suffix, the suffix would still get
> > appended. An example is "test" vs "test:[0]" using multi-format would
> > result in two tracepoints ("test:[0]" and "test:[0]:[1]" respectively
> > (assuming these are the first multi-events on the system).
> > 
> > I'm with you, we really don't want any spoofing or squatting possible.
> > By using different system names and always appending the suffix I
> > believe covers this.
> > 
> > Looking forward to hearing Steven's thoughts on this as well.
> 
> I'm leaning towards Masami's suggestion to use dots, as that won't conflict
> with special characters from bash, as '[' and ']' do.
> 

Thanks, yeah ideally we wouldn't use special characters.

I'm not picky about this. However, I did want something that clearly
allowed a glob pattern to find all versions of a given register name of
user_events by user programs that record. The dot notation will pull in
more than expected if dotted namespace style names are used.

An example is "Asserts" and "Asserts.Verbose" from different programs.
If we tried to find all versions of "Asserts" via glob of "Asserts.*" it
will pull in "Asserts.Verbose.1" in addition to "Asserts.0".

While a glob of "Asserts.[0-9]" works when the unique ID is 0-9, it
doesn't work if the number is higher, like 128. If we ever decide to
change the ID from an integer to say hex to save space, these globs
would break.

Is there some scheme that fits the C-variable name that addresses the
above scenarios? Brackets gave me a simple glob that seemed to prevent a
lot of this ("Asserts.\[*\]" in this case).

Are we confident that we always want to represent the ID as a base-10
integer vs a base-16 integer? The suffix will be ABI to ensure recording
programs can find their events easily.

Thanks,
-Beau

> -- Steve