Re: [PATCH v9 01/18] perf env: Add perf_env__e_machine helper and use in perf_env__arch

Namhyung Kim
2026-05-04T01:35:57Z

On Sat, May 02, 2026 at 05:22:31PM -0700, Ian Rogers wrote:
> Add a helper that lazily computes the e_machine and falls back to
> EM_HOST. Use the perf_env's arch to compute the e_machine if
> available. Use a binary search for some efficiency in this, but handle
> somewhat complex duplicate rules. Switch perf_env__arch to be derived
> the e_machine for consistency. This switches arch from being uname
> derived to matching that of the perf binary (via EM_HOST). Update
> session to use the helper, which may mean using EM_HOST when no
> threads are available. This also updates the perf data file header
> that gets the e_machine/e_flags from the session.
>
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
[SNIP]
> diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
> index f30e48eb3fc3..f1ae61392cce 100644
> --- a/tools/perf/util/header.c
> +++ b/tools/perf/util/header.c
> @@ -379,21 +379,28 @@ static int write_osrelease(struct feat_fd *ff,
> return do_write_string(ff, uts.release);
> }
>
> -static int write_arch(struct feat_fd *ff,
> - struct evlist *evlist __maybe_unused)
> +static int write_arch(struct feat_fd *ff, struct evlist *evlist)
> {
> struct utsname uts;
> - int ret;
> + const char *arch = NULL;
>
> - ret = uname(&uts);
> - if (ret < 0)
> - return -1;
> + if (evlist->session) {
> + /* Force the computation in the perf_env of the e_machine of the threads. */
> + perf_session__e_machine(evlist->session, /*e_flags=*/NULL);
> + arch = perf_env__arch(perf_session__env(evlist->session));
> + }
>
> - return do_write_string(ff, uts.machine);
> + if (!arch) {
> + int ret = uname(&uts);
> +
> + if (ret < 0)
> + return -1;
> + arch = uts.machine;
> + }
> + return do_write_string(ff, arch);
> }
>
> -static int write_e_machine(struct feat_fd *ff,
> - struct evlist *evlist __maybe_unused)
> +static int write_e_machine(struct feat_fd *ff, struct evlist *evlist)
> {
> /* e_machine expanded from 16 to 32-bits for alignment. */
> uint32_t e_flags;
> @@ -2684,10 +2691,18 @@ static int process_##__feat(struct feat_fd *ff, void *data __maybe_unused) \
> FEAT_PROCESS_STR_FUN(hostname, hostname);
> FEAT_PROCESS_STR_FUN(osrelease, os_release);
> FEAT_PROCESS_STR_FUN(version, version);
> -FEAT_PROCESS_STR_FUN(arch, arch);
> FEAT_PROCESS_STR_FUN(cpudesc, cpu_desc);
> FEAT_PROCESS_STR_FUN(cpuid, cpuid);
>
> +static int process_arch(struct feat_fd *ff, void *data __maybe_unused)
> +{
> + free(ff->ph->env.arch);
> + ff->ph->env.arch = do_read_string(ff);
> + if (!ff->ph->env.arch)
> + return -ENOMEM;
> + return 0;
> +}

Isn't it same as FEAT_PROCESS_STR_FUN()?

> +
> static int process_e_machine(struct feat_fd *ff, void *data __maybe_unused)
> {
> int ret;
> diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
> index fe0de2a0277f..3e64db2d27c2 100644
> --- a/tools/perf/util/session.c
> +++ b/tools/perf/util/session.c
> @@ -3023,14 +3023,19 @@ uint16_t perf_session__e_machine(struct perf_session *session, uint32_t *e_flags
> return EM_HOST;
> }
>
> + /*
> + * Is the env caching an e_machine? If not we want to compute from the
> + * more accurate threads.
> + */
> env = perf_session__env(session);
> - if (env && env->e_machine != EM_NONE) {
> - if (e_flags)
> - *e_flags = env->e_flags;
> -
> - return env->e_machine;
> - }
> + if (env && env->e_machine != EM_NONE)
> + return perf_env__e_machine(env, e_flags);
>
> + /*
> + * Compute from threads, note this is more accurate than
> + * perf_env__e_machine that falls back on EM_HOST and doesn't consider
> + * mixed 32-bit and 64-bit threads.
> + */

I'm curious if it's always better. If EM_HOST is 64-bit and the first
thread in a session happens to be 32-bit. Then resulting e_machine
would be 32- bit, right? Is that what we want?

Thanks,
Namhyung

> machines__for_each_thread(&session->machines,
> perf_session__e_machine_cb,
> &args);
> @@ -3048,10 +3053,9 @@ uint16_t perf_session__e_machine(struct perf_session *session, uint32_t *e_flags
>
> /*
> * Couldn't determine from the perf_env or current set of
> - * threads. Default to the host.
> + * threads. Potentially use logic that uses the arch string otherwise
> + * default to the host. Don't cache in the perf_env in case later
> + * threads indicate a better ELF machine type.
> */
> - if (e_flags)
> - *e_flags = EF_HOST;
> -
> - return EM_HOST;
> + return perf_env__e_machine_nocache(env, e_flags);
> }
> --
> 2.54.0.545.g6539524ca2-goog
>