perf trace: Always print return value for syscalls returning a pid

JIRA: https://issues.redhat.com/browse/RHEL-78308

upstream
========
commit c7a48ea
Author: Anubhav Shelat <ashelat@redhat.com>
Date: Thu Apr 3 12:04:12 2025 -0400

description
===========
The syscalls that were consistently observed were set_robust_list and
rseq. This is because perf cannot find their child process.

This change ensures that the return value is always printed.

Before:
     0.256 ( 0.001 ms): set_robust_list(head: 0x7f09c77dba20, len: 24)                        =
     0.259 ( 0.001 ms): rseq(rseq: 0x7f09c77dc0e0, rseq_len: 32, sig: 1392848979)             =
After:
     0.270 ( 0.002 ms): set_robust_list(head: 0x7f0bb14a6a20, len: 24)                        = 0
     0.273 ( 0.002 ms): rseq(rseq: 0x7f0bb14a70e0, rseq_len: 32, sig: 1392848979)             = 0

Committer notes:

As discussed in the thread in the Link: tag below, these two don't
return a pid, but for syscalls returning one, we need to print the
result and if we manage to find the children in 'perf trace' data
structures, then print its name as well.

Fixes: 11c8e39 ("perf trace: Infrastructure to show COMM strings for syscalls returning PIDs")
    Reviewed-by: Howard Chu <howardchu95@gmail.com>
    Signed-off-by: Anubhav Shelat <ashelat@redhat.com>
    Acked-by: Namhyung Kim <namhyung@kernel.org>
    Cc: Adrian Hunter <adrian.hunter@intel.com>
    Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
    Cc: Dapeng Mi <dapeng1.mi@linux.intel.com>
    Cc: Ian Rogers <irogers@google.com>
    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: James Clark <james.clark@linaro.org>
    Cc: Jiri Olsa <jolsa@kernel.org>
    Cc: Kan Liang <kan.liang@linux.intel.com>
    Cc: Mark Rutland <mark.rutland@arm.com>
    Cc: Michael Petlan <mpetlan@redhat.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Link: https://lore.kernel.org/r/20250403160411.159238-2-ashelat@redhat.com
    Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

Signed-off-by: Anubhav Shelat <ashelat@redhat.com>

Author: AnubhavS1

tools/perf/builtin-trace.c

@@ -2868,8 +2868,8 @@ errno_print: {
 	else if (sc->fmt->errpid) {
 		struct thread *child = machine__find_thread(trace->host, ret, ret);
 
+		fprintf(trace->output, "%ld", ret);
 		if (child != NULL) {
-			fprintf(trace->output, "%ld", ret);
 			if (thread__comm_set(child))
 				fprintf(trace->output, " (%s)", thread__comm_str(child));
 			thread__put(child);