Re: [RFCv2 1/4] perf stat: balance opening and reading events

[Jiri Olsa <jolsa@redhat.com>]

On Fri, Jul 15, 2016 at 11:08:10AM +0100, Mark Rutland wrote:
> In create_perf_stat_counter, when a target CPU has not been provided, we
> call __perf_evsel__open with empty_cpu_map, and open a single FD per
> thread. However, in read_counter we assume that we opened events for
> the product of threads and CPUs described in the evsel's cpu_map.
> 
> Thus, if an evsel has a cpu_map with more than one entry, we will
> attempt to access FDs that we didn't open. This could result in a number
> of problems (e.g. blocking while reading from STDIN if the fd memory
> happened to be initialised to zero).
> 
> This is problematic for systems were a logical CPU PMU covers some
> arbitrary subset of CPUs. The cpu_map of any evsel for that PMU will be
> initialised based on the cpumask exposed through sysfs, even if the user
> requests per-thread events.
> 
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: linux-kernel@vger.kernel.org

Acked-by: Jiri Olsa <jolsa@kernel.org>

thanks,
jirka

> ---
>  tools/perf/builtin-stat.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
> index ee7ada7..f3e21a2 100644
> --- a/tools/perf/builtin-stat.c
> +++ b/tools/perf/builtin-stat.c
> @@ -276,8 +276,12 @@ perf_evsel__write_stat_event(struct perf_evsel *counter, u32 cpu, u32 thread,
>  static int read_counter(struct perf_evsel *counter)
>  {
>  	int nthreads = thread_map__nr(evsel_list->threads);
> -	int ncpus = perf_evsel__nr_cpus(counter);
> -	int cpu, thread;
> +	int ncpus, cpu, thread;
> +
> +	if (target__has_cpu(&target))
> +		ncpus = perf_evsel__nr_cpus(counter);
> +	else
> +		ncpus = 1;
>  
>  	if (!counter->supported)
>  		return -ENOENT;
> -- 
> 1.9.1
>