Re: [PATCH 1/3 v2] perf stat: don't report a null stalled cycles per insn metric

[Song Liu]

> On Feb 7, 2020, at 3:06 PM, Kim Phillips <kim.phillips@xxxxxxx> 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
> 
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> Cc: Cong Wang <xiyou.wangcong@xxxxxxxxx>
> Cc: Jin Yao <yao.jin@xxxxxxxxxxxxxxx>
> Cc: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> Cc: Kim Phillips <kim.phillips@xxxxxxx>
> Cc: Song Liu <songliubraving@xxxxxx>
> Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
> Cc: linux-perf-users@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> Acked-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Fixes: 146540fb545b ("perf stat: Always separate stalled cycles per insn")
> Signed-off-by: Kim Phillips <kim.phillips@xxxxxxx>

Acked-by: Song Liu <songliubraving@xxxxxx>

Thanks for improving perf on AMD systems!