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 F14CD3BD64C for ; Wed, 27 May 2026 18:02:51 +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=1779904975; cv=none; b=LZIF8RLHs5k1q3Y3o4EV++UDEw4zXpKkM0zmoOiCPZcR+6STZHX+LXW58iG2Kviadc2HYEysPQ44kZ/exVthKA0zSxwj9KMxacDahGYcsBLA8v0zWa9F+z/ypIYfLJA23CpgURIfUFLZGiakbhXgIlmhf4jpLxaMAsyoCiKjMRo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779904975; c=relaxed/simple; bh=0vS54TfhA41MGHFsOIRIoxCyRog1e60flvDdXIdCGFs=; h=Message-ID:Subject:From:To:Date:In-Reply-To:References: Content-Type:MIME-Version; b=RZaro2qHTJsYmPo6nyjirogmNY7ptHtkQKMhvKOGOZxCV0ctWEl0eEWpC7Gt/6TZnZlIpB179kBeEG5ZA/byEdGyU/Ov3YrpzYlBVfQcjvuqE+AXZ7aM3kisJhXEEgw1AZL+elb4L/H4opMj0Jya8Kfs1PCKm7D6XWC8ww4T4aY= 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=Va9i5jX2; 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="Va9i5jX2" 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=Aof2tnm6avafhdR/iR6nC+Eqg6LFzami6E0Y/+7V9/w=; b=Va9i5jX2vKXLRZhDoMRWrKTRRt /n7wdItVmYHMCZ8hvCC1/i2H5BhJK6tVlu/YQT3KEwZ/LQsqtvF7oKPAheBCoyr9cYgom44+21ReS ClM+Qkpjsi0n5Hzm4DgCIKqYXS5I5uym/5mn4gAdBBCdUtuwmH92YExgw/I4E8RJGusf8oS7PqF+c CY7SXha1mlIwG30HiEu3G0E26NIgVbZoMMUYiynF5hyNhK88+FyuZ2tPSHYPrgASt4mZiNTYwdFfg qLMNfG+Q4YDopyKjda2tSvCh2DAww2cmjwrsTJ4zu4LeWMnnNqvdJPvHPS9YHygAMOMi2zFnvNMSo cwvXb2vQ==; Received: from 54-240-197-239.amazon.com ([54.240.197.239] helo=lhr50-en-acc-sw5.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1wSIaS-00000003CSF-3hLv; Wed, 27 May 2026 18:02:37 +0000 Message-ID: Subject: Re: [PATCH v4 5/7] timekeeping: Drive time_offset skew via per-tick ntp_error transfer 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: Wed, 27 May 2026 19:02:35 +0100 In-Reply-To: <20260525135904.126282-6-dwmw2@infradead.org> References: <20260525135904.126282-1-dwmw2@infradead.org> <20260525135904.126282-6-dwmw2@infradead.org> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-78latdIp8Qm1k6DvdpKz" 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 --=-78latdIp8Qm1k6DvdpKz Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2026-05-25 at 14:54 +0100, David Woodhouse wrote: >=20 > @@ -460,9 +486,14 @@ int second_overflow(unsigned int tkid, time64_t secs= ) > =C2=A0 /* Compute the phase adjustment for the next second */ > =C2=A0 ntpdata->tick_length =3D ntpdata->tick_length_base; > =C2=A0 > + /* > + * Set the per-tick skew rate for the tick code. This is in the > + * same units as tick_length (ns << NTP_SCALE_SHIFT), and is > + * rounded to a multiple of NTP_INTERVAL_FREQ so that the per-tick > + * division in the tick code is exact. > + */ > =C2=A0 delta =3D ntp_offset_chunk(ntpdata, ntpdata->time_offset); > - ntpdata->time_offset -=3D delta; > - ntpdata->tick_length +=3D delta; > + ntpdata->skew_delta =3D div_s64(delta, NTP_INTERVAL_FREQ) * NTP_INTERV= AL_FREQ; > =C2=A0 > =C2=A0 /* Check PPS signal */ > =C2=A0 pps_dec_valid(ntpdata); It turns out the infinite impulse response really *is* infinite because once time_offset gets low enough that ntp_offset_chunk() returns zero, that last bit of time_offset is *never* drained. And rounding down to a multiple of NTP_INTERVAL_FREQ as I do in this patch, just increases the threshold below which it stops draining. If I repost this series I'll set skew_delta to =C2=B1NTP_INTERVAL_FREQ if i= t ends up zero when time_offset isn't. Otherwise I'll do it in a follow- up. In practice it barely matters; it's *fractions* of a nanosecond. But it's annoying :) --=-78latdIp8Qm1k6DvdpKz 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 GfZI+LGqSv0YtNa54T9D1AWJy08ZKkWvfGGIXN9UFAPMJ6OLLH/UUEgFa+7KlrEvMUupDFGnnR06 aDJAwtycb8yXtILj+TvfhLFhafxroXrflspavejQkEiHjNjtHnwbZ+o43g0/yxjwnarGI3kgcak7 nnI9/8Lqpq79tLHYwLajotwLiGTB71AGN5xK+tzB+D4eN9lXayrjcszgbOv2ZCgzExQUAIt98mre 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