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

[Namhyung Kim]

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@xxxxxxxxxx>
> ---
[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
>