From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 539F7409628 for ; Tue, 19 May 2026 13:26:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779197173; cv=none; b=dH6wJVZObaDhIZFxujtJ11bp2UDLfqbtLCTjgiWkdtwVk1qQ4pqBujoTFzu21vSr3oEt8t9fVHEEjHHt+Ag79wjhaCosjIVj6ksPTZv3cM7LF43wlWz45JpSG3V5m1o7VHU29V3YmN1ibqVMDnU4PQuCrhDHOzFlrwtWuRPuKVU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779197173; c=relaxed/simple; bh=VqUVG6gJexGksfAqChi/0PsELKSWwxU0gTFhNsYhfys=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=XEMl2YchsqAifVl0krFkpGIhwmJL9ONeaBIrXlGahtQ+7bYj/j2qJwcdL4fQ3aI3YuKGeiXsQi/RFAe55G3ggpMDd8aN4zL8O1bjZrAA2+hejn4P0rWnrEfsbXgpqvWmvHzTd8mjAYH/LjUozeJbne9gIdOFb0vwBvaIXoi5KwQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=DoEvcBcH; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="DoEvcBcH" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1779197171; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=6p5HuL+NDLhx0zuT/Yz336oSYPmFXUUs4pVKQDgFlYo=; b=DoEvcBcH2Mn2RFmI1FP/nmQ4SgRJyx4W7oK+QGqtYN3WdXgDw0p37Ti5aCceaY2ifKFVlN VbZPKn0DUQYqa9gj1b06XIPQcc/OkD4zz0kb3ndfzrSTWAAsEv2NNjLLttI4vhTGPGZPs7 idPQD5ODtRKPnWmF1NO8q+YUR+/88Xc= Received: from mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-526-J22SVmDAOsuApqBFBAOEbQ-1; Tue, 19 May 2026 09:26:08 -0400 X-MC-Unique: J22SVmDAOsuApqBFBAOEbQ-1 X-Mimecast-MFC-AGG-ID: J22SVmDAOsuApqBFBAOEbQ_1779197165 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 08B97180061D; Tue, 19 May 2026 13:26:05 +0000 (UTC) Received: from localhost (unknown [10.43.135.229]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id DC74719560A3; Tue, 19 May 2026 13:25:59 +0000 (UTC) Date: Tue, 19 May 2026 15:25:57 +0200 From: Miroslav Lichvar To: David Woodhouse Cc: 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 , Julien Ridoux , Ryan Luu , linux-kernel@vger.kernel.org, David Woodhouse Subject: Re: [RFC PATCH v2 3/8] timekeeping: Clamp time_offset delta to prevent infinite tail Message-ID: References: <20260517220326.4625-1-dwmw2@infradead.org> <20260517220326.4625-4-dwmw2@infradead.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260517220326.4625-4-dwmw2@infradead.org> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 On Sun, May 17, 2026 at 10:25:40PM +0100, David Woodhouse wrote: > From: David Woodhouse > > ntp_offset_chunk() computes delta as time_offset >> (SHIFT_PLL + > time_constant), which exponentially decays toward zero but never > reaches it. This means time_offset asymptotically approaches zero > without ever completing — the clock never fully converges. That's how the NTP PLL was designed to work. It is an infinite impulse response filter. > Fix by clamping delta: > - Minimum: 20ns/sec (NTP_OFFSET_DELTA_MIN), ensuring the tail > converges in finite time I don't think that is an acceptable change of the filter. The impact could be measured on a sufficiently stable clock. To me that looks like using a wrong tool for the job. -- Miroslav Lichvar