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 2AFC33E276D for ; Tue, 19 May 2026 22:18:31 +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=1779229114; cv=none; b=kqeVgleHM60fXoleT9rN/TuRksCGw4bYHj0I1SQXWnSYEYPDu4605om8+cw7Spvcb4D13Czdt3WzNuA5hemzF2GZiasxLFAIJxhuulSLadRD6e81TqLV4pFNFrBXxI+lcO7ObEuYBOk/iYw9D9OClGH4S7KCgW/r+JD8ygAXp10= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779229114; c=relaxed/simple; bh=vj1GTJuAtp6rxodk2D1u8INeAou6l+/FF+PICZMLFcU=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=or2FT4yT6fmqMYLc6cYQP2elJrs2VE7Danx5gTY6HRceviCv1Ue0CSMpQuEDPiq9Od1MH1DkJ551RQKh5O2KRYSaIVetQ0uay3JqrAxfx6mhiJAoedx5CMV7jGs55E/Y2q3dYTkHO2sX1xuNdKr+LDKg3WbBcGoQiNtpC3KJVHI= 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=q4GpFwKM; 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="q4GpFwKM" 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:Cc:To:From:Subject:Message-ID:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=vj1GTJuAtp6rxodk2D1u8INeAou6l+/FF+PICZMLFcU=; b=q4GpFwKMWZ0ld41oKnN3CLM243 V9gKpl8FuXDoKTNdMBZ7Q4OwYMbTcz4k2/jtYheuPjhMIK4dMJK27qxD8/icdQCmxN+fnkbX4554W D1SCi+qsUm2lGExKMzaW4SNs7dy3uldWOM4PBCg7ffEcDe1NFI88+CwDjzPyupMwbX3FqzAGv9uM0 THVB3z56lU6iQ9t+myfW/CDoGkopzoEjGgPYG6ff6NhRxgDG8yUvHDyZ+znUKRwNB/ErMIKYwamqp VrhTR4mSmUKy49hhHKNDqlr0dg8Bqz6e/4H20kyh97ErgzBh9CPB9mQJDsTtNYh8AIINabiTGFFkn qmde/OpA==; Received: from 54-240-197-233.amazon.com ([54.240.197.233] helo=u09cd745991455d.ant.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1wPSlY-00000006M5N-2iwo; Tue, 19 May 2026 22:18:20 +0000 Message-ID: Subject: Re: [RFC PATCH 1/4] timekeeping: Remove xtime_remainder from ntp_error accumulation From: David Woodhouse To: John Stultz Cc: Miroslav Lichvar , Richard Cochran , Wen Gu , Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Thomas Gleixner , Stephen Boyd , Anna-Maria Behnsen , Frederic Weisbecker , Shuah Khan , Peter Zijlstra , Thomas =?ISO-8859-1?Q?Wei=DFschuh?= , Arnd Bergmann , linux-kernel@vger.kernel.org Date: Tue, 19 May 2026 23:18:19 +0100 In-Reply-To: References: <20260513210157.3410814-1-dwmw2@infradead.org> <20260513210157.3410814-2-dwmw2@infradead.org> <5b8a8117a5c45482a515d9fcf026b2a1ae7c5176.camel@infradead.org> <112efe3ec75c3a0bf3ea9d53034a7dea9bd553c7.camel@infradead.org> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-dtgUTVlRF7NQ/htGvvln" 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 --=-dtgUTVlRF7NQ/htGvvln Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Tue, 2026-05-19 at 14:54 -0700, John Stultz wrote: >=20 > > By simply adding ntp_interval and then subtracting the correct current > > value of xtime_interval, logarithmic_accumulation doesn't *need* a > > separate variable which tracks that delta. I don't quite understand why > > it ever did add it in the first place. >=20 > Again, it's due to the error caused by the granularity of the > clocksource (against the HZ interval).=C2=A0 That initial delta will > otherwise cause an accurate clock to get erroniously steered either > faster or slower when no NTP adjustments being made.=C2=A0 The subtractin= g > of that remainder is just trying to compensate for this base error. I must be confused. We *are* subtracting that remainder, not ignoring it. The value of xtime_remainder was (tick_length - xtime_interval). It's the discrepancy between the distance we *want* the clock to move in a single tick, and the distance it *does* move in a single tick at a given value of 'mult'. So we add tick_length, and we subtract xtime_interval. Thus adjusting ntp_error by the difference between them. I don't understand why the old code was then *also* subtracting a stale value of xtime_remainder which had been calculated at boot time. Am I missing something here?=20 I've updated the commit message: https://git.infradead.org/?p=3Dusers/dwmw2/linux.git;a=3Dcommitdiff;h=3D459= ebaef612 > Now, it is imperfect because it's a constant adjustment and not > proportioned to the NTP adjustments that might be later made. >=20 > And clearly there is some issue as you're having problems with it > using a fine-grained clocksource like the TSC where it really > shouldn't be a major factor. None of it matters if you have NTP running and constantly re-adjusting your clock. You can have a +50PPM drift through errors in the tracking, and NTP will just assume your oscillator is running 50PPM fast and adjust accordingly. But when you set the kernel to follow a precise y=3Dmx+c line for the TSC to real time conversion and *expect* it to do as it's told while tracking the divergence to the nanosecond... all counters are course- grained :) --=-dtgUTVlRF7NQ/htGvvln Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCD9Aw ggSOMIIDdqADAgECAhAOmiw0ECVD4cWj5DqVrT9PMA0GCSqGSIb3DQEBCwUAMGUxCzAJBgNVBAYT AlVTMRUwEwYDVQQKEwxEaWdpQ2VydCBJbmMxGTAXBgNVBAsTEHd3dy5kaWdpY2VydC5jb20xJDAi BgNVBAMTG0RpZ2lDZXJ0IEFzc3VyZWQgSUQgUm9vdCBDQTAeFw0yNDAxMzAwMDAwMDBaFw0zMTEx MDkyMzU5NTlaMEExCzAJBgNVBAYTAkFVMRAwDgYDVQQKEwdWZXJva2V5MSAwHgYDVQQDExdWZXJv a2V5IFNlY3VyZSBFbWFpbCBHMjCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBAMjvgLKj jfhCFqxYyRiW8g3cNFAvltDbK5AzcOaR7yVzVGadr4YcCVxjKrEJOgi7WEOH8rUgCNB5cTD8N/Et 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