Re: [Patch v3 2/5] perf x86/topdown: Correct leader selection with sample_read enabled

["Liang, Kan" <kan.liang@linux.intel.com>]

On 2024-07-12 1:03 p.m., Dapeng Mi wrote:
> Addresses an issue where, in the absence of a topdown metrics event
> within a sampling group, the slots event was incorrectly bypassed as
> the sampling leader when sample_read was enabled.
> 
> perf record -e '{slots,branches}:S' -c 10000 -vv sleep 1
> 
> In this case, the slots event should be sampled as leader but the
> branches event is sampled in fact like the verbose output shows.
> 
> perf_event_attr:
>   type                             4 (cpu)
>   size                             168
>   config                           0x400 (slots)
>   sample_type                      IP|TID|TIME|READ|CPU|IDENTIFIER
>   read_format                      ID|GROUP|LOST
>   disabled                         1
>   sample_id_all                    1
>   exclude_guest                    1
> ------------------------------------------------------------
> sys_perf_event_open: pid -1  cpu 0  group_fd -1  flags 0x8 = 5
> ------------------------------------------------------------
> perf_event_attr:
>   type                             0 (PERF_TYPE_HARDWARE)
>   size                             168
>   config                           0x4 (PERF_COUNT_HW_BRANCH_INSTRUCTIONS)
>   { sample_period, sample_freq }   10000
>   sample_type                      IP|TID|TIME|READ|CPU|IDENTIFIER
>   read_format                      ID|GROUP|LOST
>   sample_id_all                    1
>   exclude_guest                    1
> 
> The sample period of slots event instead of branches event is reset to
> 0.
> 
> This fix ensures the slots event remains the leader under these
> conditions.
> 
> Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
> ---
>  tools/perf/arch/x86/util/topdown.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/perf/arch/x86/util/topdown.c b/tools/perf/arch/x86/util/topdown.c
> index 49f25d67ed77..857e00cf579f 100644
> --- a/tools/perf/arch/x86/util/topdown.c
> +++ b/tools/perf/arch/x86/util/topdown.c
> @@ -1,6 +1,7 @@
>  // SPDX-License-Identifier: GPL-2.0
>  #include "api/fs/fs.h"
>  #include "util/evsel.h"
> +#include "util/evlist.h"
>  #include "util/pmu.h"
>  #include "util/pmus.h"
>  #include "util/topdown.h"
> @@ -87,11 +88,22 @@ bool arch_is_topdown_metrics(const struct evsel *evsel)
>   */
>  bool arch_topdown_sample_read(struct evsel *leader)
>  {
> +	struct evsel *evsel;
> +
>  	if (!evsel__sys_has_perf_metrics(leader))
>  		return false;
>  
> -	if (arch_is_topdown_slots(leader))
> -		return true;
> +	if (!arch_is_topdown_slots(leader))
> +		return false;
> +
> +	/*
> +	 * If slots event as leader event but no topdown metric events
> +	 * in group, slots event should still sample as leader.
> +	 */
> +	evlist__for_each_entry(leader->evlist, evsel) {
		evsel = leader->leader;
		evlist__for_each_entry_continue(leader->evlist, evsel)
			if (evsel->leader != leader->leader)
				return false;

Maybe we should limit the check in a group, rather than the entire
evlist. Something as above (not tested)?

Thanks,
Kan
> +		if (evsel != leader && arch_is_topdown_metrics(evsel))
> +			return true;
> +	}
>  
>  	return false;
>  }