linux-kernel - Re: [Patch v3 2/5] perf x86/topdown: Correct leader selection with sample

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Message-ID: <b9cd52b6-b7c3-4f9c-b8d5-6f6508465075@linux.intel.com>
Date: Tue, 13 Aug 2024 15:15:08 +0800
From: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To: "Liang, Kan" <kan.liang@...ux.intel.com>,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc: linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
 Yongwei Ma <yongwei.ma@...el.com>, Dapeng Mi <dapeng1.mi@...el.com>
Subject: Re: [Patch v3 2/5] perf x86/topdown: Correct leader selection with
 sample_read enabled


On 8/12/2024 11:18 PM, Liang, Kan wrote:
>
> 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@...ux.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)?

Good point. Would limit the check in the group.


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