From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 351B61EB5FD; Sat, 12 Sep 2026 07:23:33 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789197815; cv=none; b=OAWls6m1zdXNEjH14Hl6mQwelSnG7TiURknjWhttVZSOxANg6Zgn3pC+KgmCgFTTc8hjWByIKDK1p77z9WalWk/G2YyAsLPPMtseC2MzlgA56fvqFq/hAQInEdfp5rvqHdhgVGaSBGJM+xI6GwQQKaHlBWlyjcYhdgdqQAv0Y/Y= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789197815; c=relaxed/simple; bh=F4CPZ4tE7CSxwB29TykMxL64tGUOUUYtKOebRsDfDmQ=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=tIuPEQxD8YhlDxYXhdcQAbEVzRu7ZVD2+fh0Nfu8aGuYauL0Wn581kqPzkV8lB+dhNYKyC7UdyavlRpItdhw1X7W/UfTuVHfArRwLVhNkoegdQ1bCRjj5SzwbehehHLl2XnU9eCPoJ6VfWFh+xgKpiC8kaTDvfoftOtpLIMfIfE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=rVIFuyka; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="rVIFuyka" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9330B1F000FF; Sat, 12 Sep 2026 07:23:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1789197813; bh=J5VpKjGyRPgxBpkSrKIQ3kuOCNOZOGlEuT19UAuUPV0=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=rVIFuykawqdCM6M2JAdvOGEEcugfT+6YlXlpLMf8YhT4z4qJOf7s31NFIMaRcChGh q1RvfWbLLiLGuY8jqAsdRoJ7WeExqiEmg7sbakDSTZOCb7Qu+hmxk7vPgJBXBdRuHT FgLJ9ZmpMW2ZeZKLvP2MvRGx5M1cNR6RdtIVIBhc= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, David Woodhouse , Thomas Gleixner , John Stultz , Sasha Levin Subject: [PATCH 7.2 0252/1815] timekeeping: Account for monotonicity adjustment in ntp_error Date: Sat, 12 Sep 2026 08:33:22 +0200 Message-ID: <20260912065654.898817132@linuxfoundation.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260912065648.999753832@linuxfoundation.org> References: <20260912065648.999753832@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 7.2-stable review patch. If anyone has any objections, please let me know. ------------------ From: David Woodhouse [ Upstream commit b7befd6d91207cf3f4cecd68fea0c212093906cf ] timekeeping_apply_adjustment() modifies xtime_nsec to ensure monotonicity when mult changes: xtime_nsec -= offset This ensures that the time reported to userspace does not jump when the multiplier is adjusted from one tick to the next. However, the ntp_error accumulator which tracks the difference between intended and actual clock position was not being updated to reflect this additional discrepancy. An earlier attempt at this compensation existed as: ntp_error -= (interval - offset) << ntp_error_shift but was removed in commit c2cda2a5bda9 ("timekeeping/ntp: Don't align NTP frequency adjustments to ticks") because it was a major source of NTP error. That's because (interval - offset) was wrong: the subtraction of "interval" prematurely accounted for the changed xtime_interval of the next tick, which would be correctly accounted in the next accumulation anyway — a double subtraction. What is actually needed is just the "offset" part: ntp_error must be told that xtime_nsec moved by "offset" without a corresponding change in the intended position. For the normal ±1 mult dithering this is negligible (the adjustments cancel over time), but for larger mult changes — such as when an external reference clock sets a new frequency — the one-time uncompensated offset is significant. Fix by adjusting ntp_error by the correct amount: ntp_error += offset << ntp_error_shift This keeps ntp_error consistent with the actual xtime_nsec position after the adjustment, and ensures the discrepancy is correctly smoothed away over time and the clock returns to where it should have been. Fixes: c2cda2a5bda9 ("timekeeping/ntp: Don't align NTP frequency adjustments to ticks") Signed-off-by: David Woodhouse Signed-off-by: Thomas Gleixner Assisted-by: Kiro:claude-opus-4.6-1m Acked-by: John Stultz Link: https://patch.msgid.link/20260621220051.1030462-3-dwmw2@infradead.org Signed-off-by: Sasha Levin --- kernel/time/timekeeping.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index 49f81336aae21..55ebf2703f1bf 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -2390,6 +2390,11 @@ static __always_inline void timekeeping_apply_adjustment(struct timekeeper *tk, * xtime_nsec_2 = xtime_nsec_1 - offset * Which simplifies to: * xtime_nsec -= offset + * + * When subtracting offset from xtime_nsec, the same amount + * (in appropriate units) has to be added to ntp_error, in + * order to correctly track the delta between the time + * reported in xtime_nsec, and the intended time. */ if ((mult_adj > 0) && (tk->tkr_mono.mult + mult_adj < mult_adj)) { /* NTP adjustment caused clocksource mult overflow */ @@ -2400,6 +2405,7 @@ static __always_inline void timekeeping_apply_adjustment(struct timekeeper *tk, tk->tkr_mono.mult += mult_adj; tk->xtime_interval += interval; tk->tkr_mono.xtime_nsec -= offset; + tk->ntp_error += offset << tk->ntp_error_shift; } /* -- 2.53.0