Re: [PATCH v2 2/5] perf record: Prevent override of attr->sample_period for libpfm4 events

From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Ian Rogers <irogers@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>, Andrii Nakryiko <andriin@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@chromium.org>,
	Adrian Hunter <adrian.hunter@intel.com>,
	Andi Kleen <ak@linux.intel.com>,
	Athira Rajeev <atrajeev@linux.vnet.ibm.com>,
	linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
	bpf@vger.kernel.org, Stephane Eranian <eranian@google.com>
Subject: Re: [PATCH v2 2/5] perf record: Prevent override of attr->sample_period for libpfm4 events
Date: Wed, 29 Jul 2020 15:54:39 -0300	[thread overview]
Message-ID: <20200729185439.GC433799@kernel.org> (raw)
In-Reply-To: <20200728085734.609930-3-irogers@google.com>

Em Tue, Jul 28, 2020 at 01:57:31AM -0700, Ian Rogers escreveu:
> From: Stephane Eranian <eranian@google.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@google.com>
> Reviewed-by: Ian Rogers <irogers@google.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