Re: [PATCH v2 4/5] perf record: Don't clear event's period if set by a term

Adrian Hunter
2020-08-04T14:49:00Z

On 4/08/20 4:33 pm, Ian Rogers wrote:
> On Tue, Aug 4, 2020 at 3:08 AM Adrian Hunter <adrian.hunter@intel.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@google.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
>>>
>>