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

> + 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;


> + 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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