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 B41352C21C5; Fri, 19 Jun 2026 20:57:54 +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=1781902679; cv=none; b=TOLBOnS8kkMNEHvLKV33/7yT6FQhWIQBs2LNNy60RnDX4qX3YETiWeRkUxZnEvd93NFSlqxoSk2CkZUOfxT77a5xuQahFl1oilTNlITAFoc+ahyzr4kgQ4qb7hkgnn2o6Oj1Rc3q49FWRTb+TMYT2Y6+FHA0/WAcPkSn0NB+hBg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781902679; c=relaxed/simple; bh=zb/zwiu6U1Je/QI5KdWKTdnm/EawPEKRqNP1ppJPp2M=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=rgcX3ljPa65bPLYa9HdMwvqH4ku9+ZMQVIco0NDlAQWMUXV0/KOlIl2Itq8ZhCbr/RzDbcHcDzX5cJR2Q0GhEF/uakpzlaUSzVJR/75xiC4zw5B9DWUq6sFeZABcHjlkfh1T8GcwksVFMmvQcvl4vR6xZwW/Jyux/drlgdFirI0= 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=wI6tXNsB; 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="wI6tXNsB" 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=zb/zwiu6U1Je/QI5KdWKTdnm/EawPEKRqNP1ppJPp2M=; b=wI6tXNsBDI2gyGsoLzuiGamiRu K0EVY0oR8ZaHkuXBqzUe/tEBC6davvhLrt/+ykB9vQ0ZhLSn3jJ1k6/j7zpXKrgLDMhx44dN6ZMV+ i0bWDnxz1yvUhl/ctuBuayW50KUQeQq4ueCCudc1fyDJxc5E2/6avEyIMN2/ZFEGI7xxVvApN3e7p UY2RSHxHUduhgZHYyHZAOf5liQ3gt+0y3Nyivz45M6lkDNSPr6p/v4C5bfKoMndn8L0MDRRuRn0sQ bOrCbC+KP9yuVuPcBhV+uHFiIO8VDIzkx0RttWnmCTTz1gmxupufaYWYqgmpxVdkAa0L6OmPs3Oct UprZnvrQ==; Received: from [2001:8b0:10b:5:c1ae:7fe3:825c:da7d] (helo=u09cd745991455d.ant.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1wagHe-0000000GpZJ-2opZ; Fri, 19 Jun 2026 20:57:51 +0000 Message-ID: <4f985ff0ffb3142d8b6dabf2be21a4667bb20494.camel@infradead.org> Subject: Re: [RFC] Enabling CONFIG_NTP_PPS for NOHZ by adding ntp_error to system_time_snapshot From: David Woodhouse To: Thomas Gleixner , John Stultz , Stephen Boyd , Miroslav Lichvar , Richard Cochran , linux-kernel@vger.kernel.org, netdev@vger.kernel.org Cc: Rodolfo Giometti , Alexander Gordeev Date: Fri, 19 Jun 2026 21:57:50 +0100 In-Reply-To: <87v7beb7s3.ffs@fw13> References: <3616fc9718614bf11915569599038a5bcb268c02.camel@infradead.org> <87h5myd56x.ffs@fw13> <02564e5f0b6be4aeb6198af87b46269963985768.camel@infradead.org> <87v7beb7s3.ffs@fw13> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-RTNOOZVDWb2yyaly0Mk0" User-Agent: Evolution 3.60.3 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 --=-RTNOOZVDWb2yyaly0Mk0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, 2026-06-19 at 22:21 +0200, Thomas Gleixner wrote: > On Fri, Jun 19 2026 at 16:34, David Woodhouse wrote: > > On Fri, 2026-06-19 at 15:34 +0200, Thomas Gleixner wrote: > > >=20 > > > This formatting makes my brain hurt. Can you please split that out in= to > > > a separate function? > >=20 > > Yep. There's also a potential error there =E2=80=94 an *additional* dis= crepancy > > comes from the enforced monotonicity that timekeeping_cycles_to_ns() > > applies (the case where it just returns tkr->xtime_nsec >> tkr_shift). > >=20 > > I couldn't work out if I cared about the clocksource-is-non-monotonic > > casse, and even if I did, what I should do about it. >=20 > I think the right thing is just to ignore it. Yeah, that was basically my conclusion; I had just meant to *mention* it when posting the RFC. > The problem is very narrow and mostly related to the historically badly > synchronized TSC between sockets. The TSC_ADJUST fixup is obviously > error prone as it adjusts only to the point where the error is not > longer observable. But in the update transition phase it can result in > time going backwards because the readout on the other CPU is slightly > behind tk::tkr_mono::cycles_last. That happens only once in a while and > we talk about a very low single digit number of TSC cycles. >=20 > > I also wasn't sure if this should be a new CLOCK_REALTIME_NONMONOTONIC > > or something like that, such that e.g. PTP clients could *ask* for it. >=20 > Hell no! That was not about the above clocksource nonsense; that was the question of what the caller (in my example case, the vmclock PTP snapshot) should *do* with the reported error value. If I just unconditionally "correct" the CLOCK_REALTIME values then that's arguably an ABI change. We're silently reporting something *different* to what we did before. Maybe that's OK... as I said, in the PPS case we can justify it and just call it a bug fix? Or maybe we want a way for callers (not of ktime_get_snapshot_id() itself, but *their* callers) to *ask* for the "corrected" value instead. I happened to call that CLOCK_REALTIME_NONMONOTONIC as a straw man, just because monotonicity is *one* of the reasons why we present the xtime values that we do, not always the raw "corrected" values. --=-RTNOOZVDWb2yyaly0Mk0 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 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