Re: [PATCH v5 1/3] perf sched: Suppress latency table output when trace samples are missing

On Fri, Jul 31, 2026 at 02:59:23PM -0700, Namhyung Kim wrote:
> Hello,
> 
> On Thu, Jul 30, 2026 at 02:54:14PM -0400, Aaron Tomlin wrote:
> > When 'perf sched latency' is executed on a perf.data file that lacks
> > tracepoint samples (i.e., a file recorded without the -R flag or
> > containing only non-tracepoint events), perf_session__has_traces()
> > correctly outputs an error message. However, perf_sched__read_events()
> > subsequently falls through and returns 0 (success).
> > 
> > Consequently, caller functions such as perf_sched__lat() assume event
> > processing succeeded and proceed to render empty latency header tables
> > and total summary statistics.
> > 
> > Fix this behaviour by ensuring perf_sched__read_events() aborts early and
> > returns a suitable error code when perf_session__has_traces() evaluates
> > to false.
> > 
> > For pipe mode streams, event attributes are received dynamically during
> > event processing, meaning session->evlist is not populated prior to
> > perf_session__process_events(). To handle pipe input correctly:
> >   - Register the missing .attr, .tracing_data, .build_id, and .feature
> >     callbacks in cmd_sched()
> > 
> >   - Promote the handlers array to file-scope (latency_handlers[]) and
> >     invoke evlist__set_tracepoints_handlers() dynamically inside
> >     perf_sched__process_tracepoint_sample() when evsel->handler is NULL
> > 
> >   - Perform the trace check post-processing when handling pipe data
> 
> Can you please split the pipe mode changes into a separate commit?
> 
> > 
> > Additionally, validate thread__get_runtime() against NULL in
> > map_switch_event() to prevent potential null-pointer dereferences.
> 
> This is small enough to be squashed.

Hi Namhyung,

Absolutely, will do.

Kind regards,
-- 
Aaron Tomlin