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 674AB386453 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=1781573287; cv=none; b=gk4Ktx34uxyYYU8syQEF+MfnlcwmdPl4dQd0OFpWarwjrDDZdkrKRQj7lp+TvRMPBujYkHo2dnoxHu97zpk5z/Q2QMqFhVoeyECzLsZkZshL8Ce0zdbWkOeNCKYfccphn2UL5B9ZgXJJWf2jPPiPvTVd0wKh9A+JA1Zng3pUnEE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781573287; c=relaxed/simple; bh=d1l9O0Ursk9irEjdNKSCcOl8mlqjfaRX/z6ly5Y6G2k=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Content-Type; b=tp5UiD96YevZo27gSLRyApZ+GVpqkuFcq6RpsHBJzqC8HgTq1hsYV0MmX+IAMwsvX6hoF7qBtPNQlWoRByY40cMdJpNobQrvy5FQqRfu+ntHxxxHKwuSwr6TV99n5aVbo2orqUn2/rc7hgcLo8YpUDawfT6HV1aPH/xNitm4faY= 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 5a478bee46e88-307625ee07fso13987861eec.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=Wvpl/sv3l2aClBFw+SYuvhKKZR9IRFIfEurLW6MlW6eC1TXXqU1YpqK//GIy6ggpRf hMPNDzpBmxTFAXasKLac4+xQmuOe9IxfPRBN4atG2qqBxi5wBysRGzYrc7J2ezZeAjEv T887Wm9CjDubGL5q3ehY/NQxhkl632o/xNTDk+IHSXM50B6APu3apOBUO43bkSYghVHS J41uG2pRIMe2PqXh9Hola1nKOZSZnqyVtfL2nDz4IpR6l1KGgAro3MzLEc9p3DFOOPSg yNWAqlX7TIul9wA7ePkFMJE4EV9O4JHJrZ4y7FFBb4LhkpNOY9hxaVcFwO/0diZnL2xR iLXA== X-Forwarded-Encrypted: i=1; AFNElJ9F2pC8qgVIHKKpSJ918nDeDDV53se5R1QyPjYNfshSauziK726GIUnBcOI/3GwEnf9nFJIOqeFp1P53so=@vger.kernel.org X-Gm-Message-State: AOJu0Yw4kcSG6V9Ru2xCi8K+e9pqHdiz9pRkfS74nQfaixrWAxPe89Fz 7wm5Yk7qw6Ru09gJjyIi+9m9mkUZPD2kRyDCX6yf0cZaaIVXmk1VbCUFDRybWaE6ZE9m7RbmvRr ZDhC2iIL8jg== 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-kernel@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 != "