From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D025119DFA2; Mon, 4 Aug 2025 00:27:18 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1754267238; cv=none; b=u3Q2GdIsPcI69U5OR+/XtF1jC8mTspfRfVE36cKsedSKi8bCbHGkjtI8HOZAjtlZIAnZ1sCx+UhwC5EOMpJLvl6A8pkpZIKEb39Tj4si717ujVGJlI6u+J6V3Tm21RsLwfErDYZO0xJ7MAw0IGbnN3hhxXmcnqp2KcyfiGzhQI0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1754267238; c=relaxed/simple; bh=c2+mqsFgTr/4k91XA1KSm9C9W++CrNEM1EGVhoym5qA=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version:Content-Type; b=MbsDoTAS5Wtq7cwI9j8+0KcEufRfoFhKVD0NHv1IMHK29xCSG4n3wL5Fb5M5QArq7haDxSlog062qLNpcXJjy5fAZZszfSw8twTxzbMu0/QjhwAFEgYcFK+8cpYIiYAnGsHUdDzWwo72rnJLj8XFswhO0sdycr3+N8U9sQErCDI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=MvjTDb7k; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="MvjTDb7k" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 76142C4CEF0; Mon, 4 Aug 2025 00:27:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1754267238; bh=c2+mqsFgTr/4k91XA1KSm9C9W++CrNEM1EGVhoym5qA=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=MvjTDb7kZkGLYwb4QFuMnBtmEoZUm2rcDlreS6I2aoxoOcIJx4kClXJgrQePb3Ur0 TYxTzQY8z9FkyKDAiwZ3AVZWnfTHSFLTJobJEm2U/E7zbIR1ymCPFmHGOp9foeri5b sIyn4OhssiTwJYi/fX13Ye50kmQaguZ+iM/J74Z/YSMejhwpplL2fKutP7M2mqKG5v WbwtLa/SHgBWOVEUNFj+VMXc7KFu1SGKfu/dxTSrxNEJU2iUbWX9+mgKdlNVa8+7Bx tw/bP9vgQI8iz2TXvmK6+MBQX1OVyeWk/up8qhL83L8IZ7N4ZZVN51y1WKZjxjs7lA FJvEXrzwpd0Og== From: Sasha Levin To: patches@lists.linux.dev, stable@vger.kernel.org Cc: "Gautham R. Shenoy" , Shuah Khan , Sasha Levin , wangfushuai@baidu.com, herongguang@linux.alibaba.com Subject: [PATCH AUTOSEL 6.16 75/85] pm: cpupower: Fix the snapshot-order of tsc,mperf, clock in mperf_stop() Date: Sun, 3 Aug 2025 20:23:24 -0400 Message-Id: <20250804002335.3613254-75-sashal@kernel.org> X-Mailer: git-send-email 2.39.5 In-Reply-To: <20250804002335.3613254-1-sashal@kernel.org> References: <20250804002335.3613254-1-sashal@kernel.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-stable: review X-Patchwork-Hint: Ignore X-stable-base: Linux 6.16 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit From: "Gautham R. Shenoy" [ Upstream commit cda7ac8ce7de84cf32a3871ba5f318aa3b79381e ] In the function mperf_start(), mperf_monitor snapshots the time, tsc and finally the aperf,mperf MSRs. However, this order of snapshotting in is reversed in mperf_stop(). As a result, the C0 residency (which is computed as delta_mperf * 100 / delta_tsc) is under-reported on CPUs that is 100% busy. Fix this by snapshotting time, tsc and then aperf,mperf in mperf_stop() in the same order as in mperf_start(). Link: https://lore.kernel.org/r/20250612122355.19629-2-gautham.shenoy@amd.com Signed-off-by: Gautham R. Shenoy Signed-off-by: Shuah Khan Signed-off-by: Sasha Levin --- LLM Generated explanations, may be completely bogus: **Backport Status: YES** This commit should be backported to stable kernel trees for the following reasons: 1. **Clear bug fix with user impact**: The commit fixes a calculation bug in the cpupower tool where C0 residency (processor active time) is under-reported on 100% busy CPUs. This directly affects users monitoring CPU performance and power states. 2. **Simple and contained fix**: The change is minimal - it only reorders three function calls in `mperf_stop()` to match the order in `mperf_start()`: - Before: `mperf_measure_stats()` → `mperf_get_tsc()` → `clock_gettime()` - After: `clock_gettime()` → `mperf_get_tsc()` → `mperf_measure_stats()` 3. **Low risk of regression**: The fix simply ensures consistent snapshot ordering between start and stop measurements. This is a logical correction that aligns the measurement methodology without introducing new functionality. 4. **Clear root cause**: The bug occurs because the reversed snapshot order causes timing skew in the C0 residency calculation (`delta_mperf * 100 / delta_tsc`). When snapshots are taken in different orders at start vs stop, the time windows don't align properly, leading to incorrect calculations. 5. **Affects monitoring tool accuracy**: While this is a userspace tool bug rather than a kernel bug, cpupower is an important diagnostic tool shipped with the kernel. Incorrect C0 residency reporting could mislead system administrators and developers analyzing CPU performance. 6. **No architectural changes**: The fix doesn't change any data structures, APIs, or fundamental behavior - it simply corrects the measurement sequence. The commit meets stable tree criteria as it fixes a real bug affecting users, is minimal in scope, and has very low risk of introducing new issues. tools/power/cpupower/utils/idle_monitor/mperf_monitor.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/tools/power/cpupower/utils/idle_monitor/mperf_monitor.c b/tools/power/cpupower/utils/idle_monitor/mperf_monitor.c index 73b6b10cbdd2..5ae02c3d5b64 100644 --- a/tools/power/cpupower/utils/idle_monitor/mperf_monitor.c +++ b/tools/power/cpupower/utils/idle_monitor/mperf_monitor.c @@ -240,9 +240,9 @@ static int mperf_stop(void) int cpu; for (cpu = 0; cpu < cpu_count; cpu++) { - mperf_measure_stats(cpu); - mperf_get_tsc(&tsc_at_measure_end[cpu]); clock_gettime(CLOCK_REALTIME, &time_end[cpu]); + mperf_get_tsc(&tsc_at_measure_end[cpu]); + mperf_measure_stats(cpu); } return 0; -- 2.39.5