linux-kernel - Re: [PATCH v2 2/5] perf record: Prevent override of attr->sample

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Date:   Wed, 29 Jul 2020 15:54:39 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Ian Rogers <irogers@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        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>,
        Adrian Hunter <adrian.hunter@...el.com>,
        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, Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v2 2/5] perf record: Prevent override of
 attr->sample_period for libpfm4 events

Em Tue, Jul 28, 2020 at 01:57:31AM -0700, Ian Rogers escreveu:
> From: Stephane Eranian <eranian@...gle.com>
> 
> Before:
> $ perf record -c 10000 --pfm-events=cycles:period=77777
> 
> Would yield a cycles event with period=10000, instead of 77777.

I tried the equivalent without libpfm and it works:

  $ perf record -c 10000 -e cycles/period=20000/ sleep 1
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.024 MB perf.data (23 samples) ]
  $ perf evlist -v
  cycles/period=20000/u: size: 120, { sample_period, sample_freq }: 20000, sample_type: IP|TID|TIME, read_format: ID, disabled: 1, inherit: 1, exclude_kernel: 1, exclude_hv: 1, mmap: 1, comm: 1, enable_on_exec: 1, task: 1, sample_id_all: 1, exclude_guest: 1, mmap2: 1, comm_exec: 1, ksymbol: 1, bpf_event: 1
  $
 
> This was due to an ordering issue between libpfm4 parsing
> the event string and perf record initializing the event.
 
> This patch fixes the problem by preventing override for
> events with attr->sample_period != 0 by the time
> perf_evsel__config() is invoked. This seems to have been the
> intent of the author.
> 
> Signed-off-by: Stephane Eranian <eranian@...gle.com>
> Reviewed-by: Ian Rogers <irogers@...gle.com>
> ---
>  tools/perf/util/evsel.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index 811f538f7d77..8afc24e2ec52 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -976,8 +976,7 @@ void evsel__config(struct evsel *evsel, struct record_opts *opts,
>  	 * We default some events to have a default interval. But keep
>  	 * it a weak assumption overridable by the user.
>  	 */
> -	if (!attr->sample_period || (opts->user_freq != UINT_MAX ||
> -				     opts->user_interval != ULLONG_MAX)) {
> +	if (!attr->sample_period) {
>  		if (opts->freq) {
>  			attr->freq		= 1;
>  			attr->sample_freq	= opts->freq;
> -- 
> 2.28.0.163.g6104cc2f0b6-goog
> 

-- 

- Arnaldo