Re: [PATCH] perf trace: always print return value for syscalls with set errpid

[Namhyung Kim <namhyung@kernel.org>]

On Thu, Apr 03, 2025 at 12:04:12PM -0400, Anubhav Shelat wrote:
> Currently some syscalls that have errpid set to true do not print a
> return value in perf trace. 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

I'm ok with the change but I'm not sure if they are right syscalls.  It
seems neither of syscalls return a thread id.

Thanks,
Namhyung

> 
> Signed-off-by: Anubhav Shelat <ashelat@redhat.com>
> ---
>  tools/perf/builtin-trace.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/perf/builtin-trace.c b/tools/perf/builtin-trace.c
> index 6ac51925ea42..c192f0219b2b 100644
> --- a/tools/perf/builtin-trace.c
> +++ b/tools/perf/builtin-trace.c
> @@ -3005,8 +3005,8 @@ 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);
> -- 
> 2.48.1
>