Re: [RFC PATCH 1/1] libperf: evlist: Fix --cpu argument on hybrid platform

[Namhyung Kim <namhyung@kernel.org>]

On Tue, Oct 15, 2024 at 03:54:15PM +0100, James Clark wrote:
> Since the linked fixes: commit, specifying a CPU on hybrid platforms
> results in an error because Perf tries to open an extended type event
> on "any" CPU which isn't valid. Extended type events can only be opened
> on CPUs that match the type.
> 
> Before (working):
> 
>   $ perf record --cpu 1 -- true
>   [ perf record: Woken up 1 times to write data ]
>   [ perf record: Captured and wrote 2.385 MB perf.data (7 samples) ]
> 
> After (not working):
> 
>   $ perf record -C 1 -- true
>   WARNING: A requested CPU in '1' is not supported by PMU 'cpu_atom' (CPUs 16-27) for event 'cycles:P'
>   Error:
>   The sys_perf_event_open() syscall returned with 22 (Invalid argument) for event (cpu_atom/cycles:P/).
>   /bin/dmesg | grep -i perf may provide additional information.
> 
> (Ignore the warning message, that's expected and not particularly
> relevant to this issue).
> 
> This is because perf_cpu_map__intersect() of the user specified CPU (1)
> and one of the PMU's CPUs (16-27) correctly results in an empty (NULL)
> CPU map. However for the purposes of opening an event, libperf converts
> empty CPU maps into an any CPU (-1) which the kernel rejects.
> 
> Fix it by deleting evsels with empty CPU maps in the specific case where
> user requested CPU maps are evaluated.

I think there's a discussion about making default events skippable and
hide them in the output unless all of them fail.

Thanks,
Namhyung

> 
> Fixes: 251aa040244a ("perf parse-events: Wildcard most "numeric" events")
> Signed-off-by: James Clark <james.clark@linaro.org>
> ---
>  tools/lib/perf/evlist.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/lib/perf/evlist.c b/tools/lib/perf/evlist.c
> index c6d67fc9e57e..8fae9a157a91 100644
> --- a/tools/lib/perf/evlist.c
> +++ b/tools/lib/perf/evlist.c
> @@ -47,6 +47,13 @@ static void __perf_evlist__propagate_maps(struct perf_evlist *evlist,
>  		 */
>  		perf_cpu_map__put(evsel->cpus);
>  		evsel->cpus = perf_cpu_map__intersect(evlist->user_requested_cpus, evsel->own_cpus);
> +
> +		/*
> +		 * Empty cpu lists would eventually get opened as "any" so remove
> +		 * genuinely empty ones before they're opened in the wrong place.
> +		 */
> +		if (perf_cpu_map__is_empty(evsel->cpus))
> +			perf_evlist__remove(evlist, evsel);
>  	} else if (!evsel->own_cpus || evlist->has_user_cpus ||
>  		(!evsel->requires_cpu && perf_cpu_map__has_any_cpu(evlist->user_requested_cpus))) {
>  		/*
> @@ -80,11 +87,11 @@ static void __perf_evlist__propagate_maps(struct perf_evlist *evlist,
>  
>  static void perf_evlist__propagate_maps(struct perf_evlist *evlist)
>  {
> -	struct perf_evsel *evsel;
> +	struct perf_evsel *evsel, *n;
>  
>  	evlist->needs_map_propagation = true;
>  
> -	perf_evlist__for_each_evsel(evlist, evsel)
> +	list_for_each_entry_safe(evsel, n, &evlist->entries, node)
>  		__perf_evlist__propagate_maps(evlist, evsel);
>  }
>  
> -- 
> 2.34.1
>