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Date: Mon, 20 Jan 2020 10:29:28 +0100
From: Jiri Olsa <jolsa@...hat.com>
To: Kim Phillips <kim.phillips@....com>
Cc: Arnaldo Carvalho de Melo <acme@...hat.com>,
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@...nel.org>,
Namhyung Kim <namhyung@...nel.org>,
Cong Wang <xiyou.wangcong@...il.com>,
Andi Kleen <ak@...ux.intel.com>,
Jin Yao <yao.jin@...ux.intel.com>,
Kan Liang <kan.liang@...ux.intel.com>,
Song Liu <songliubraving@...com>,
Davidlohr Bueso <dave@...olabs.net>,
linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] perf stat: don't report a null stalled cycles per
insn metric
On Wed, Jan 15, 2020 at 04:29:49PM -0600, Kim Phillips wrote:
> For data collected on machines with front end stalled cycles supported,
> such as found on modern AMD CPU families, commit 146540fb545b ("perf
> stat: Always separate stalled cycles per insn") introduces a new line
> in CSV output with a leading comma that upsets some automated scripts.
> Scripts have to use "-e ex_ret_instr" to work around this issue, after
> upgrading to a version of perf with that commit.
>
> We could add "if (have_frontend_stalled && !config->csv_sep)"
> to the not (total && avg) else clause, to emphasize that CSV users
> are usually scripts, and are written to do only what is needed, i.e.,
> they wouldn't typically invoke "perf stat" without specifying an
> explicit event list.
>
> But - let alone CSV output - why should users now tolerate a constant
> 0-reporting extra line in regular terminal output?:
>
> BEFORE:
>
> $ sudo perf stat --all-cpus -einstructions,cycles -- sleep 1
>
> Performance counter stats for 'system wide':
>
> 181,110,981 instructions # 0.58 insn per cycle
> # 0.00 stalled cycles per insn
> 309,876,469 cycles
>
> 1.002202582 seconds time elapsed
>
> The user would not like to see the now permanent
> "0.00 stalled cycles per insn" line fixture, as it gives
> no useful information.
>
> So this patch removes the printing of the zeroed stalled cycles
> line altogether, almost reverting the very original commit fb4605ba47e7
> ("perf stat: Check for frontend stalled for metrics"), which seems
> like it was written to normalize --metric-only column output
> of common Intel machines at the time: modern Intel machines
> have ceased to support the genericised frontend stalled metrics AFAICT.
>
> AFTER:
>
> $ sudo perf stat --all-cpus -einstructions,cycles -- sleep 1
>
> Performance counter stats for 'system wide':
>
> 244,071,432 instructions # 0.69 insn per cycle
> 355,353,490 cycles
>
> 1.001862516 seconds time elapsed
>
> Output behaviour when stalled cycles is indeed measured is not affected
> (BEFORE == AFTER):
>
> $ sudo perf stat --all-cpus -einstructions,cycles,stalled-cycles-frontend -- sleep 1
>
> Performance counter stats for 'system wide':
>
> 247,227,799 instructions # 0.63 insn per cycle
> # 0.26 stalled cycles per insn
> 394,745,636 cycles
> 63,194,485 stalled-cycles-frontend # 16.01% frontend cycles idle
>
> 1.002079770 seconds time elapsed
looks reasonable to me, Andi, are you ok with this?
Acked-by: Jiri Olsa <jolsa@...hat.com
thanks,
jirka
>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
> Cc: Jiri Olsa <jolsa@...nel.org>
> Cc: Namhyung Kim <namhyung@...nel.org>
> Cc: Cong Wang <xiyou.wangcong@...il.com>
> Cc: Andi Kleen <ak@...ux.intel.com>
> Cc: Jin Yao <yao.jin@...ux.intel.com>
> Cc: Kan Liang <kan.liang@...ux.intel.com>
> Cc: Kim Phillips <kim.phillips@....com>
> Cc: Song Liu <songliubraving@...com>
> Cc: Davidlohr Bueso <dave@...olabs.net>
> Cc: linux-perf-users@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Fixes: 146540fb545b ("perf stat: Always separate stalled cycles per insn")
> Signed-off-by: Kim Phillips <kim.phillips@....com>
> ---
> tools/perf/util/stat-shadow.c | 6 ------
> 1 file changed, 6 deletions(-)
>
> diff --git a/tools/perf/util/stat-shadow.c b/tools/perf/util/stat-shadow.c
> index 2c41d47f6f83..90d23cc3c8d4 100644
> --- a/tools/perf/util/stat-shadow.c
> +++ b/tools/perf/util/stat-shadow.c
> @@ -18,7 +18,6 @@
> * AGGR_NONE: Use matching CPU
> * AGGR_THREAD: Not supported?
> */
> -static bool have_frontend_stalled;
>
> struct runtime_stat rt_stat;
> struct stats walltime_nsecs_stats;
> @@ -144,7 +143,6 @@ void runtime_stat__exit(struct runtime_stat *st)
>
> void perf_stat__init_shadow_stats(void)
> {
> - have_frontend_stalled = pmu_have_event("cpu", "stalled-cycles-frontend");
> runtime_stat__init(&rt_stat);
> }
>
> @@ -853,10 +851,6 @@ void perf_stat__print_shadow_stats(struct perf_stat_config *config,
> print_metric(config, ctxp, NULL, "%7.2f ",
> "stalled cycles per insn",
> ratio);
> - } else if (have_frontend_stalled) {
> - out->new_line(config, ctxp);
> - print_metric(config, ctxp, NULL, "%7.2f ",
> - "stalled cycles per insn", 0);
> }
> } else if (perf_evsel__match(evsel, HARDWARE, HW_BRANCH_MISSES)) {
> if (runtime_stat_n(st, STAT_BRANCHES, ctx, cpu) != 0)
> --
> 2.24.1
>
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