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 0AAE63FE348; Sat, 12 Sep 2026 09:51:03 +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=1789206665; cv=none; b=T0s9+XwfFBXW7j7V4A3CkrFgfoOUHQym6nPlDuJn0vI73pMDQSE5UZqFo+R1MXYMbLG3VN2OXgIunfhKibmevz/d8G2OSx0X6EI6uAU7rRXKHf5C+RvT4FaaqzjNTBtBlAI3tefZ/nTGFKB1zLosDsMaW5eOVjzzFHG/wLiLdfg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789206665; c=relaxed/simple; bh=pl00WbNQ7yW781mrbGB18Niq6lZVCrY2JkDjEJl4I10=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=C1Z6FQ3GznVm0THpjj175WdB9tR58fUDj1OZUIUpqnh09EcfLQeCxY+facFb4aKh3ZoZaVLcR3WWGl1yc81nqTItMQrzbkb9V2Pp+sEYuPm7ueJzjII+G9vhUUf+dHTlLjODqgE0Gwi6t5mQ8OdPwe9mRYoyALNnQ+PI+6CggVM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=VD/U8/D3; 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="VD/U8/D3" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D1D841F000FF; Sat, 12 Sep 2026 09:51:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1789206662; bh=50OWI+b5KL0EhcefMIpYs0aYgz7CjrJ80X8SHK/IYHo=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=VD/U8/D37rCBEtTYVuxuItzRT0A/sVTS4fmRolRrycnejMEWWIO5Plc9Kj4OoJbPW Aj33VfwN9sqhI33cGClFvoLrD/rQpgRAZIR2h8U4AMJVfg2SiAlQtwCmkAYIU5ha90 ezLegv4A9L9TEZusMPlw9QGv0oxTrY84a6wNVwvU= 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 6.18 0253/1518] timekeeping: Account for monotonicity adjustment in ntp_error Date: Sat, 12 Sep 2026 08:40:22 +0200 Message-ID: <20260912065629.206906214@linuxfoundation.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260912065623.398859879@linuxfoundation.org> References: <20260912065623.398859879@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 6.18-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 e6060d9392d09..03408b5fcf695 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -2140,6 +2140,11 @@ 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 */ @@ -2150,6 +2155,7 @@ 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