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

[Namhyung Kim <namhyung@kernel.org>]

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.

Thanks,
Namhyung