From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-dy1-f202.google.com (mail-dy1-f202.google.com [74.125.82.202]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6738E37C0FC for ; Tue, 16 Jun 2026 01:28:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=74.125.82.202 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781573289; cv=none; b=rc/tgeHrZCCIqV3pMUzJy255g4UoRzkdAHLaCbKK0kDVEv964rS6kvSTn2b9b2+RvlazWwbd1lzgWrq+Wr3n76xK0ivLtXcedDjXo1lNpSWlEimsBzpOri2R26TNXEY9K+TWkyNbied7A1TFCWMIPGWgfZjOa6oq44qKZXg9jP8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781573289; c=relaxed/simple; bh=d1l9O0Ursk9irEjdNKSCcOl8mlqjfaRX/z6ly5Y6G2k=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Content-Type; b=VJpMf3/elF49NLwuuIw6F5cnKL5adG4CgkA33hVWVI4VLs97t4DIRjZqa8kD8bZ3zMHgfPH4qZ5GMRyttPd8Z+nvfHj70nAqwgGprBvnCwbtX7UrAea4ynIH/rZyoB5uiqm96rN92SgVnit74LDYSSD/C1HfEEVeEymFQIh17vk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--irogers.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=mzgOc0yC; arc=none smtp.client-ip=74.125.82.202 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--irogers.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="mzgOc0yC" Received: by mail-dy1-f202.google.com with SMTP id 5a478bee46e88-307625ee07fso13987864eec.1 for ; Mon, 15 Jun 2026 18:28:06 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1781573286; x=1782178086; darn=vger.kernel.org; h=to:from:subject:message-id:references:mime-version:in-reply-to:date :from:to:cc:subject:date:message-id:reply-to; bh=wlWTMBTGUUs0KMAWTHlPpE8gLWyfQrfVnfm0X/gpu54=; b=mzgOc0yCP2mqo8cU+mPNCvAXrZ4B1bQ8Dccsx0KiNFRtMXQiFtHhPiD1tSVJdRNSOH sBnIpEysxI2HGk8Li4l80+9dXkMRUAK967tN/Oq4C6PxtzPl9HqbgKWSpO71i1Onjnsl oglGCJ2ttuuCsoz3uHgFiMfOJBfmOxa2CnXM7YrYo+E+n1BaszlpyyOZZ3+GU4c97qW3 +hjxRz+b/7iiMSYks0vX9vqETCcgxNNf1yTvQwXmIwlfqWBWqp7FHbplmTV3ZS9sXWSD U7ijl2dGvo90iVvJYoRRqGMm6Qx5aeMJ90NNrhozrJX7ayOOq+ly6lfTQosgcudNoAVN oe5A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1781573286; x=1782178086; h=to:from:subject:message-id:references:mime-version:in-reply-to:date :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=wlWTMBTGUUs0KMAWTHlPpE8gLWyfQrfVnfm0X/gpu54=; b=pKljgM0gnIJY41+AE0a5zHli5yJf8AY7b/SNCantDRkjQwy0Iudl0MQ7BQqrqSbLpv ewDi/pKXgvpvYjAIvUvvSq23uui5cnx9pLK/IXx+mIqnZDOSh05FOOsYQWqj6IqoM3LU 3ZlQrVlLAiGYXY7dNVk5AFMZ1ljf1JdLwbX7jfJmM8lqXliDJD3fZM0WK2u5x4+PU4sQ Fd9MmB/qMpOUyqwJ72TGVPwdkKImj5Qz1KUIqIpvX2cP3sjYjmGk4tCujakvKtNaD/jS iRUX2C4rFl5ofuTyapai/i1nTTLLN0b+ZeRAIGKuHwmfo2r9orE3HBWzyfNZ6JemmncO QSrw== X-Forwarded-Encrypted: i=1; AFNElJ9On2iqtagbzzJQuul69tky2AQ643XmtIMvittLCdquhCVON6uLhHyp5vzvSNqBkImwbTGXiL13B1do8Bvo1Zsj@vger.kernel.org X-Gm-Message-State: AOJu0YxOrhlbH717JBVzaYwD/HCKbO23vxn4LEejT5u3PsTqdx0zapcG IPaWXbLNDcXfrI/EooyManJ8yTXlsak4tgdUZgAk9DKtUGDBDffdSl84zxN/CcpxHp486xSBR+p Lkv9XcpSe+g== X-Received: from dyjt21.prod.google.com ([2002:a05:7300:7b15:b0:304:ee5b:e1a5]) (user=irogers job=prod-delivery.src-stubby-dispatcher) by 2002:a05:7300:6ca5:b0:304:cefc:5fde with SMTP id 5a478bee46e88-308202087c5mr8867844eec.26.1781573285195; Mon, 15 Jun 2026 18:28:05 -0700 (PDT) Date: Mon, 15 Jun 2026 18:27:40 -0700 In-Reply-To: <20260616012744.4049193-1-irogers@google.com> Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260616012744.4049193-1-irogers@google.com> X-Mailer: git-send-email 2.54.0.1136.gdb2ca164c4-goog Message-ID: <20260616012744.4049193-9-irogers@google.com> Subject: [PATCH v1 08/12] perf tests: Fix flakiness in BPF counters test on hybrid systems From: Ian Rogers To: Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim , Jiri Olsa , Ian Rogers , Adrian Hunter , James Clark , Thomas Falcon , Leo Yan , Thomas Richter , linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org Content-Type: text/plain; charset="UTF-8" The `perf stat --bpf-counters test` fails intermittently on hybrid architectures or systems with dynamic frequency scaling (DVFS). This happens because the test workload (`sqrtloop`) runs for a fixed 1-second duration, and the CPU frequency can scale dynamically between idle and maximum frequency. As the first run runs on a cold CPU and the second run runs on a warmed-up CPU (or vice versa), the number of instructions executed in 1 second differs by up to 2.2x, violating the comparison tolerance. Also, when running as root, BPF tracepoints and scheduling programs trigger frequently. Since standard `perf stat -e instructions` measures both user and kernel space instructions, it counts BPF helper and program execution overheads, whereas the BPF counters themselves do not self-measure. This introduces a large kernel-space instruction count discrepancy between standard and BPF counters. Fix these issues by: 1. Switching the workload to a strictly deterministic, iteration-based workload: `awk 'BEGIN { for (i=0; i<10000000; i++) sum+=i }'`. We pin the workload to a single random allowed CPU using `taskset -c $CPU` via a bash array. 2. Restricting the counted event to user-space only (`instructions:u` or `/u`). 3. Tightening the comparison tolerance from 20% to 15%. These modifications isolate the measurements to user-space instructions of the deterministic loop, which executes a virtually identical number of instructions on both runs (with less than 0.001% variation), eliminating Dynamic Frequency Scaling (DVFS), kernel scheduling noise, and BPF helper self-measurement overheads. Fixes: 2c0cb9f56020 ("perf test: Add a shell test for 'perf stat --bpf-counters' new option") Signed-off-by: Ian Rogers --- tools/perf/tests/shell/stat_bpf_counters.sh | 27 +++++++++++++-------- 1 file changed, 17 insertions(+), 10 deletions(-) diff --git a/tools/perf/tests/shell/stat_bpf_counters.sh b/tools/perf/tests/shell/stat_bpf_counters.sh index 35463358b273..2f01608c95e3 100755 --- a/tools/perf/tests/shell/stat_bpf_counters.sh +++ b/tools/perf/tests/shell/stat_bpf_counters.sh @@ -4,21 +4,25 @@ set -e -workload="perf test -w sqrtloop" +CPU=0 +if command -v shuf >/dev/null 2>&1; then + CPU=$(shuf -i 0-$(($(nproc) - 1)) -n 1 2>/dev/null || echo 0) +fi +workload=(taskset -c "$CPU" awk 'BEGIN { for (i=0; i<10000000; i++) sum+=i }') -# check whether $2 is within +/- 20% of $1 +# check whether $2 is within +/- 15% of $1 compare_number() { first_num=$1 second_num=$2 - # upper bound is first_num * 120% - upper=$(expr $first_num + $first_num / 5 ) - # lower bound is first_num * 80% - lower=$(expr $first_num - $first_num / 5 ) + # upper bound is first_num * 115% + upper=$(expr $first_num + $first_num \* 3 / 20 ) + # lower bound is first_num * 85% + lower=$(expr $first_num - $first_num \* 3 / 20 ) if [ $second_num -gt $upper ] || [ $second_num -lt $lower ]; then - echo "The difference between $first_num and $second_num are greater than 20%." + echo "The difference between $first_num and $second_num are greater than 15%." exit 1 fi } @@ -41,11 +45,12 @@ check_counts() test_bpf_counters() { printf "Testing --bpf-counters " - base_instructions=$(perf stat --no-big-num -e instructions -- $workload 2>&1 | \ + base_instructions=$(perf stat --no-big-num -e instructions:u -- "${workload[@]}" 2>&1 | \ awk -v i=0 -v c=0 '/instructions/ { \ if ($1 != " 0) printf "%.0f", c; else print "&1 | \ + bpf_instructions=$(perf stat --no-big-num --bpf-counters -e instructions:u \ + -- "${workload[@]}" 2>&1 | \ awk -v i=0 -v c=0 '/instructions/ { \ if ($1 != " 0) printf "%.0f", c; else print "&1) + stat_output=$(perf stat --no-big-num \ + -e instructions/name=base_instructions/u,instructions/name=bpf_instructions/bu \ + -- "${workload[@]}" 2>&1) base_instructions=$(echo "$stat_output"| \ awk -v i=0 -v c=0 '/base_instructions/ { \ if ($1 != "