From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (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 433072E5B1B for ; Fri, 29 May 2026 14:41:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780065687; cv=none; b=ueohnpuD7NAUtmxMsTGaFXSdwRYa9/iCnb9g4iBSZCHzzSmOIcG6YJn/Pv2TmYI4yZjpC/kI6xke16sd7n6wTE0F8LDCbPHMc5w6rRt9Xne1ASvoOHdrURpgBZ/6sEFHB7RGCCEn+HKq4AtOobrzQw6qD80fY4S2Q9iD8DwO6Wc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780065687; c=relaxed/simple; bh=8VHy0EY2/XaSDmuBIcb5hfaj+NV8v6J7lQuRaseIIOk=; h=Message-ID:Subject:From:To:Date:In-Reply-To:References: Content-Type:MIME-Version; b=IHWjY5trpHTLSh/PCekQTKjBNLqOYewV8xlByf+B3kcKJCyqY4skGdh79riOCWDiA+G54UiomwBL1KIVSEuqn0SNLl2iGjXr2/0wzVS7lNYJsJ79Yv7XCImxjio1LR41BAC8r42J7C7uMzkVSBhbmDfsX7bsk7vi1CXIiXCXIuI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=casper.srs.infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=tIIc3q5h; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=casper.srs.infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="tIIc3q5h" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=MIME-Version:Content-Type:References: In-Reply-To:Date:To:From:Subject:Message-ID:Sender:Reply-To:Cc: Content-Transfer-Encoding:Content-ID:Content-Description; bh=MpPU8x8pYyVrQ9iaDHDs0BuzDyq0mwr2J9+Gy7ihtDA=; b=tIIc3q5hK469bp41DgH7M9NVgd /PVFzvr0XluZ98yKwXpiac0/xOPt5T2W2EbneVOVyNvh5KrYuLAXUPQ8emmNQpSdNDJ7JBeZW23Rp iPUti6vo+lu1mrK5i2Xhv68daDH1LkSx0ZvqblRP7/SNJ+OM6SAkr93cKTDQGP/GzCQGx14qUGHxd sgpUUTxERBwT+XipxSBrNSb/LLjqhB4zav6StgrqLDi9qQJBjgNOXJBVLZKZyBXV4szrFAcmeTcdg 5QRFrVxLgXekWfmt3weKiBGxFzlUdZknlWL9hAhS/VqxX9rI7MnzVt3RnVHxJfnpTWIOspSYBT1aW 6zc++n9g==; Received: from 54-240-197-238.amazon.com ([54.240.197.238] helo=edge-cache-117.e-fra50.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1wSyOZ-00000006Gyz-3Gr1; Fri, 29 May 2026 14:41:07 +0000 Message-ID: <36370cd28ed4f588aebbc19a4d7bb5e88107ae3e.camel@infradead.org> Subject: Re: [PATCH v4 2/7] timekeeping: Remove xtime_remainder from ntp_error accumulation From: David Woodhouse To: Richard Cochran , Wen Gu , Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , John Stultz , Thomas Gleixner , Stephen Boyd , Anna-Maria Behnsen , Frederic Weisbecker , Shuah Khan , Peter Zijlstra , Thomas =?ISO-8859-1?Q?Wei=DFschuh?= , Arnd Bergmann , Miroslav Lichvar , Julien Ridoux , Ryan Luu , linux-kernel@vger.kernel.org Date: Fri, 29 May 2026 15:41:06 +0100 In-Reply-To: <20260525135904.126282-3-dwmw2@infradead.org> References: <20260525135904.126282-1-dwmw2@infradead.org> <20260525135904.126282-3-dwmw2@infradead.org> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-Kc6qY7vcq+kOS61YfgBl" User-Agent: Evolution 3.52.3-0ubuntu1.1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by casper.infradead.org. See http://www.infradead.org/rpr.html --=-Kc6qY7vcq+kOS61YfgBl Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2026-05-25 at 14:54 +0100, David Woodhouse wrote: > From: David Woodhouse >=20 > The ntp_error accumulator tracks the difference between the time actually > reported to consumers in xtime, and the *intended* time. The former is > subject to a sawtooth effect due to the quantisation of 'mult', which > means that it actually advances by 'xtime_interval' each tick, while > the intended clock advances by 'ntp_tick'. >=20 > By dithering between adjacent integer values of 'mult' which result in > an 'xtime_interval' slightly higher/lower than the intended tick length, > the advancement of xtime is kept on average to the intended rate. >=20 > The accounting should therefore adjust ntp_error by adding ntp_tick and > subtracting xtime_interval on each tick. >=20 > Since commit a386b5af8edd ("time: Compensate for rounding on > odd-frequency clocksources") the value subtracted has been > (xtime_interval + xtime_remainder), which is wrong. The effect is a > systematic drift whose magnitude depends on the value of xtime_remainder > and the NTP frequency correction. NTP masks this by continuously > adjusting the frequency to compensate, but with a fixed frequency (or an > external reference clock like vmclock), the drift is exposed. >=20 > The value of xtime_remainder actually does represent the difference > between the tick period and xtime_interval, so simply adding it instead > of (+ tick length - xtime_remainder) might have made sense... except > that it's only calculated once at boot time, so it's inaccurate anyway. > So just kill it with fire. >=20 > Also remove it from the mult computation in timekeeping_adjust(), which > used it to offset the division for the same (incorrect) reason. >=20 > Fixes: a386b5af8edd ("time: Compensate for rounding on odd-frequency cloc= ksources") > Signed-off-by: David Woodhouse > Assisted-by: Kiro:claude-opus-4.6-1m > --- Sashiko asks, "Does this removal cause tk->ntp_error to diverge towards negative infinity when NTP is inactive and cycle_interval rounds up?" And Sashiko is right; it *does*. But it's also *correct* to do so. Because tk_setup_internals() currently rounds 'mult' to the *nearest* value, instead of rounding *down* like timekeeping_adjust() does. So the dithering that timekeeping_adjust() does between mult and mult+1 can't actually achieve the desired result, and we *do* just drift further and further away from where we should be, because 'mult' is one too many than it should be. The values chosen for mult/mult+1 are supposed to be either *side* of the desired average period. Any adjtimex() call which sets the frequency will fix this, because mult *is* correctly calculated in timekeeping_adjust(). I'll fix that pre-existing bug in a separate patch, in v5. Something like this in tk_setup_internals() should do it: - /* Go back from cycles -> shifted ns */ - tk->xtime_interval =3D interval * clock->mult; - tk->raw_interval =3D interval * clock->mult; - tk->tkr_mono.mult =3D clock->mult; + tk->tkr_mono.mult =3D div64_u64(ntpinterval, tk- >cycle_interval); + /* Go back from cycles -> shifted ns */ + tk->xtime_interval =3D interval * tk->tkr_mono.mult; + tk->raw_interval =3D interval * tk->tkr_raw.mult; --=-Kc6qY7vcq+kOS61YfgBl Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCD9Aw ggSOMIIDdqADAgECAhAOmiw0ECVD4cWj5DqVrT9PMA0GCSqGSIb3DQEBCwUAMGUxCzAJBgNVBAYT AlVTMRUwEwYDVQQKEwxEaWdpQ2VydCBJbmMxGTAXBgNVBAsTEHd3dy5kaWdpY2VydC5jb20xJDAi BgNVBAMTG0RpZ2lDZXJ0IEFzc3VyZWQgSUQgUm9vdCBDQTAeFw0yNDAxMzAwMDAwMDBaFw0zMTEx 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