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Date:   Tue, 4 Aug 2020 17:48:26 +0300
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Ian Rogers <irogers@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Athira Rajeev <atrajeev@...ux.vnet.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v2 4/5] perf record: Don't clear event's period if set by
 a term

On 4/08/20 4:33 pm, Ian Rogers wrote:
> On Tue, Aug 4, 2020 at 3:08 AM Adrian Hunter <adrian.hunter@...el.com> wrote:
>>
>> On 28/07/20 11:57 am, Ian Rogers wrote:
>>> If events in a group explicitly set a frequency or period with leader
>>> sampling, don't disable the samples on those events.
>>>
>>> Prior to 5.8:
>>> perf record -e '{cycles/period=12345000/,instructions/period=6789000/}:S'
>>
>> Might be worth explaining this use-case some more.
>> Perhaps add it to the leader sampling documentation for perf-list.
>>
>>> would clear the attributes then apply the config terms. In commit
>>> 5f34278867b7 leader sampling configuration was moved to after applying the
>>> config terms, in the example, making the instructions' event have its period
>>> cleared.
>>> This change makes it so that sampling is only disabled if configuration
>>> terms aren't present.
>>>
>>> Fixes: 5f34278867b7 ("perf evlist: Move leader-sampling configuration")
>>> Signed-off-by: Ian Rogers <irogers@...gle.com>
>>> ---
>>>  tools/perf/util/record.c | 28 ++++++++++++++++++++--------
>>>  1 file changed, 20 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/tools/perf/util/record.c b/tools/perf/util/record.c
>>> index a4cc11592f6b..01d1c6c613f7 100644
>>> --- a/tools/perf/util/record.c
>>> +++ b/tools/perf/util/record.c
>>> @@ -2,6 +2,7 @@
>>>  #include "debug.h"
>>>  #include "evlist.h"
>>>  #include "evsel.h"
>>> +#include "evsel_config.h"
>>>  #include "parse-events.h"
>>>  #include <errno.h>
>>>  #include <limits.h>
>>> @@ -38,6 +39,9 @@ static void evsel__config_leader_sampling(struct evsel *evsel, struct evlist *ev
>>>       struct perf_event_attr *attr = &evsel->core.attr;
>>>       struct evsel *leader = evsel->leader;
>>>       struct evsel *read_sampler;
>>> +     struct evsel_config_term *term;
>>> +     struct list_head *config_terms = &evsel->config_terms;
>>> +     int term_types, freq_mask;
>>>
>>>       if (!leader->sample_read)
>>>               return;
>>> @@ -47,16 +51,24 @@ static void evsel__config_leader_sampling(struct evsel *evsel, struct evlist *ev
>>>       if (evsel == read_sampler)
>>>               return;
>>>
>>> +     /* Determine the evsel's config term types. */
>>> +     term_types = 0;
>>> +     list_for_each_entry(term, config_terms, list) {
>>> +             term_types |= 1 << term->type;
>>> +     }
>>>       /*
>>> -      * Disable sampling for all group members other than the leader in
>>> -      * case the leader 'leads' the sampling, except when the leader is an
>>> -      * AUX area event, in which case the 2nd event in the group is the one
>>> -      * that 'leads' the sampling.
>>> +      * Disable sampling for all group members except those with explicit
>>> +      * config terms or the leader. In the case of an AUX area event, the 2nd
>>> +      * event in the group is the one that 'leads' the sampling.
>>>        */
>>> -     attr->freq           = 0;
>>> -     attr->sample_freq    = 0;
>>> -     attr->sample_period  = 0;
>>> -     attr->write_backward = 0;
>>> +     freq_mask = (1 << EVSEL__CONFIG_TERM_FREQ) | (1 << EVSEL__CONFIG_TERM_PERIOD);
>>> +     if ((term_types & freq_mask) == 0) {
>>
>> It would be nicer to have a helper e.g.
>>
>>         if (!evsel__have_config_term(evsel, FREQ) &&
>>             !evsel__have_config_term(evsel, PERIOD)) {
> 
> Sure. The point of doing it this way was to avoid repeatedly iterating
> over the config term list.

But perhaps it is premature optimization

> 
>>> +             attr->freq           = 0;
>>> +             attr->sample_freq    = 0;
>>> +             attr->sample_period  = 0;
>>
>> If we are not sampling, then maybe we should also put here:
>>
>>                 attr->write_backward = 0;
>>
>>> +     }
>>
>> Then, if we are sampling this evsel shouldn't the backward setting
>> match the leader? e.g.
>>
>>         if (attr->sample_freq)
>>                 attr->write_backward = leader->core.attr.write_backward;
> 
> Perhaps that should be a follow up change? This change is trying to
> make the behavior match the previous behavior.

Sure

> 
> Thanks,
> Ian
> 
>>> +     if ((term_types & (1 << EVSEL__CONFIG_TERM_OVERWRITE)) == 0)
>>> +             attr->write_backward = 0;
>>>
>>>       /*
>>>        * We don't get a sample for slave events, we make them when delivering
>>>
>>

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