Re: [RFC PATCH v3 4/6] sched, tracing: reorganize fields of switch event struct

[Ze Gao]

Thanks for clarifying this ! Steven. This is really helpful.

Regards,
Ze

On Tue, Aug 1, 2023 at 10:33 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Tue, 1 Aug 2023 13:46:50 +0200
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> > On Tue, Aug 01, 2023 at 05:01:22PM +0800, Ze Gao wrote:
> > > Report priorities in 'short' and prev_state in 'int' to save
> > > some buffer space. And also reorder the fields so that we take
> > > struct alignment into consideration to make the record compact.
> > >
> > > Suggested-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> >
> > I don't see a single line describing the effort you've done to audit
> > consumers of this tracepoint.
> >
> > *IF* you're wanting to break this tracepoint ABI, because seriously
> > that's what it is, then you get to invest the time and effort to audit
> > the users.
>
> The known major users that I am aware of is raesdaemon,
> powertop/latencytop, perf, trace-cmd and some bpf tools. The bpf tooling is
> known to update per kernel. The others all use libtraceevent that can
> handle this change.
>
> What other tools are there? There's Perfetto, but it also looks at tracefs
> to examine where the values are. There's LTTng, but I believe it uses the
> raw tracepoint directly and doesn't look at the layout of the ftrace/perf
> buffers.
>
> All other tooling I am slightly aware of uses libtracefs and libtraceveent,
> as I've been giving many talks on how to use those libraries.
>
> -- Steve