linux-kernel - Re: [PATCH v3] perf test: Allow tolerance for leader sampling test

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Message-ID: <Z_hkF6R9giaDGQZD@google.com>
Date: Thu, 10 Apr 2025 17:36:39 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Thomas Richter <tmricht@...ux.ibm.com>
Cc: linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
	linux-perf-users@...r.kernel.org, acme@...nel.org,
	ctshao@...gle.com, rogers@...gle.com, agordeev@...ux.ibm.com,
	gor@...ux.ibm.com, sumanthk@...ux.ibm.com, hca@...ux.ibm.com,
	Ian Rogers <irogers@...gle.com>
Subject: Re: [PATCH v3] perf test: Allow tolerance for leader sampling test

Hello,

On Thu, Apr 10, 2025 at 10:55:22AM +0200, Thomas Richter wrote:
> V3: Added check for missing samples as suggested by Chun-Tse.
> V2: Changed bc invocation to return 0 on success and 1 on error.
> 
> There is a known issue that the leader sampling is inconsistent, since
> throttle only affect leader, not the slave. The detail is in [1]. To
> maintain test coverage, this patch sets a tolerance rate of 80% to
> accommodate the throttled samples and prevent test failures due to
> throttling.
> 
> [1] lore.kernel.org/20250328182752.769662-1-ctshao@...gle.com
> 
> Signed-off-by: Chun-Tse Shao <ctshao@...gle.com>
> Suggested-by: Ian Rogers <irogers@...gle.com>
> Suggested-by: Thomas Richter <tmricht@...ux.ibm.com>
> Tested-by: Thomas Richter <tmricht@...ux.ibm.com>
> Signed-off-by: Thomas Richter <tmricht@...ux.ibm.com>
> ---
>  tools/perf/tests/shell/record.sh | 33 ++++++++++++++++++++++++++------
>  1 file changed, 27 insertions(+), 6 deletions(-)
> 
> diff --git a/tools/perf/tests/shell/record.sh b/tools/perf/tests/shell/record.sh
> index ba8d873d3ca7..0075ffe783ad 100755
> --- a/tools/perf/tests/shell/record.sh
> +++ b/tools/perf/tests/shell/record.sh
> @@ -238,22 +238,43 @@ test_leader_sampling() {
>      err=1
>      return
>    fi
> +  perf script -i "${perfdata}" | grep brstack > $script_output
> +  # Check if the two instruction counts are equal in each record.
> +  # However, the throttling code doesn't consider event grouping. During throttling, only the
> +  # leader is stopped, causing the slave's counts significantly higher. To temporarily solve this,
> +  # let's set the tolerance rate to 80%.
> +  # TODO: Revert the code for tolerance once the throttling mechanism is fixed.
>    index=0
> -  perf script -i "${perfdata}" > $script_output
> +  valid_counts=0
> +  invalid_counts=0
> +  tolerance_rate=0.8
>    while IFS= read -r line
>    do
> -    # Check if the two instruction counts are equal in each record
>      cycles=$(echo $line | awk '{for(i=1;i<=NF;i++) if($i=="cycles:") print $(i-1)}')
>      if [ $(($index%2)) -ne 0 ] && [ ${cycles}x != ${prev_cycles}x ]
>      then
> -      echo "Leader sampling [Failed inconsistent cycles count]"
> -      err=1
> -      return
> +      invalid_counts=$(($invalid_counts+1))
> +    else
> +      valid_counts=$(($valid_counts+1))
>      fi
>      index=$(($index+1))
>      prev_cycles=$cycles
>    done < $script_output
> -  echo "Basic leader sampling test [Success]"
> +  total_counts=$(bc <<< "$invalid_counts+$valid_counts")
> +  if (( $(bc <<< "$total_counts <= 0") ))
> +  then
> +    echo "Leader sampling [No sample generated]"
> +    err=1
> +    return
> +  fi
> +  isok=$(bc <<< "scale=2; if (($invalid_counts/$total_counts) < (1-$tolerance_rate)) { 0 } else { 1 };")

Is 'scale=2' really needed?  Does something similar to the above like

  if (( $(bc <<< "($invalid_counts / $total_counts) < (1 - $tolerance_rate)") ))

work?

Thanks,
Namhyung


> +  if [ $isok -eq 1 ]
> +  then
> +     echo "Leader sampling [Failed inconsistent cycles count]"
> +     err=1
> +  else
> +    echo "Basic leader sampling test [Success]"
> +  fi
>  }
>  
>  test_topdown_leader_sampling() {
> -- 
> 2.49.0
>