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

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Date:   Tue, 4 Aug 2020 13:08:07 +0300
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Ian Rogers <irogers@...gle.com>,
        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>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     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 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)) {

> +		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
>