From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 458ACC10F0E for ; Tue, 9 Apr 2019 09:22:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1215320833 for ; Tue, 9 Apr 2019 09:22:04 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="Q2BVTu8v" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726825AbfDIJWC (ORCPT ); Tue, 9 Apr 2019 05:22:02 -0400 Received: from mail-pl1-f193.google.com ([209.85.214.193]:35511 "EHLO mail-pl1-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726370AbfDIJWC (ORCPT ); Tue, 9 Apr 2019 05:22:02 -0400 Received: by mail-pl1-f193.google.com with SMTP id w24so9055578plp.2 for ; Tue, 09 Apr 2019 02:22:02 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:subject:to:cc:references:in-reply-to:mime-version :user-agent:message-id:content-transfer-encoding; bh=YC1feqKlZEVMS4YoQTZ4Ne8qnxJnVw8mIrsaa5cJO1s=; b=Q2BVTu8v8RaHnw+RJ33EBqlCDvyN95SyH3iRtjS8e4OLg8n5Gpk4pe1wrDeAE21o2o w1HJZyQlewnGhXF/s0SRjvE79c2HsbEncGinhM/CWLG2GCcSRZu0K3vecOBS0pcoFriP iewULm1VV6OVRzQBTtbJVPPdjenUYGAsmhQuBquy89RT/16LgWcT0S+8QCvTz41qeTjD Zfas6U/5vUymoXmWz3kWZIsghciz3iiIeq42iXMEh9+Kyb/iwL1rjDHOodhWyKyYPrc0 QMB09SN6ovwiYYbZhQ1gVGoqq9XlVYHuZ2Dn/WkEo5ca3CNrZfOGcjwqxCI+G9+hGfyF 59ug== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:subject:to:cc:references:in-reply-to :mime-version:user-agent:message-id:content-transfer-encoding; bh=YC1feqKlZEVMS4YoQTZ4Ne8qnxJnVw8mIrsaa5cJO1s=; b=Fkas76JNK4paii3FjOQbuNTJGBBlSmQoTlXlPDysSKqNV+NTegPZsZRS5DsGS8bLvX YctWzdKUCrSnbtMVFpiI0uF5CCOJjTPmmIhR9+imvMoPFtvBXOW4XYDTzyXCEvFtYtxq IsaEHG87mjufduHeAPJ2tSJfnci8SvYQOiB5lQUgFwXcaakmZpbG7ekAcTQu3r7NaTYG GcpYp+f1ctkzF2DP/PBElzEopulTBxyfq39SZjHiAlRNof8HmQ809Y4Gi/1py1jfTxzd 1vQ44cTTgGMuG3rIZBfOodavY3D8DxAHW65Zxo56d0+cHTazbJWMd4hHzVamerBMiGaH eYmw== X-Gm-Message-State: APjAAAWLg3bQB1geM+ILzYTO9Y7O9XhKrCSGggaORbvCEFR/7HrHPMn3 Jn3/dEsrCrBU2DgZDFp3bA4= X-Google-Smtp-Source: APXvYqw6HVklM7gvOhRdGBZMNKTtPCqzE49p+QBftb7VgZ2pwtboNH00CRR6e+uxB7qHHTTEJS8klQ== X-Received: by 2002:a17:902:28e9:: with SMTP id f96mr14996001plb.190.1554801721726; Tue, 09 Apr 2019 02:22:01 -0700 (PDT) Received: from localhost ([58.84.79.14]) by smtp.gmail.com with ESMTPSA id b19sm21883248pgb.51.2019.04.09.02.22.00 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Tue, 09 Apr 2019 02:22:01 -0700 (PDT) Date: Tue, 09 Apr 2019 19:21:54 +1000 From: Nicholas Piggin Subject: Re: [PATCH 0/4] Allow CPU0 to be nohz full To: Thomas Gleixner Cc: Frederic Weisbecker , linux-kernel@vger.kernel.org, Ingo Molnar , Peter Zijlstra , "Rafael J . Wysocki" References: <20190404120704.18479-1-npiggin@gmail.com> <1554393113.wbjxx9ccdx.astroid@bobo.none> In-Reply-To: MIME-Version: 1.0 User-Agent: astroid/0.14.0 (https://github.com/astroidmail/astroid) Message-Id: <1554800737.v126tflazd.astroid@bobo.none> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Thomas Gleixner's on April 6, 2019 3:54 am: > On Fri, 5 Apr 2019, Nicholas Piggin wrote: >> Thomas Gleixner's on April 5, 2019 12:36 am: >> > On Thu, 4 Apr 2019, Nicholas Piggin wrote: >> >=20 >> >> I've been looking at ways to fix suspend breakage with CPU0 as a >> >> nohz CPU. I started looking at various things like allowing CPU0 >> >> to take over do_timer again temporarily or allowing nohz full >> >> to be stopped at runtime (that is quite a significant change for >> >> little real benefit). The problem then was having the housekeeping >> >> CPU go offline. >> >>=20 >> >> So I decided to try just allowing the freeze to occur on non-zero >> >> CPU. This seems to be a lot simpler to get working, but I guess >> >> some archs won't be able to deal with this? Would it be okay to >> >> make it opt-in per arch? >> >=20 >> > It needs to be opt in. x86 will fall on its nose with that. >>=20 >> Okay I can add that. >>=20 >> > Now the real interesting question is WHY do we need that at all? >>=20 >> Why full nohz for CPU0? Basically this is how their job system was >> written and used, testing nohz full was a change that came much later=20 >> as an optimisation. >>=20 >> I don't think there is a fundamental reason an equivalent system >> could not be made that uses a different CPU for housekeeping, but I >> was assured the change would be quite difficult for them. >>=20 >> If we can support it, it seems nice if you can take a particular >> configuration and just apply nohz_full to your application processors >> without any other changes. >=20 > This wants an explanation in the patches. Okay. > And patch 4 has in the changelog: >=20 > nohz_full has been successful at significantly reducing jitter for a > large supercomputer customer, but their job control system requires CP= U0 > to be for housekeeping. >=20 > which just makes me dazed and confused :) >=20 > Other than some coherent explanation and making it opt in, I don't think > there is a fundamental issue with that. I will try to make the changelogs less jibberish then :) Thanks, Nick =