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=-5.9 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=no 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 DD1CFC433DF for ; Sat, 25 Jul 2020 17:36:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BA6FC2065F for ; Sat, 25 Jul 2020 17:36:40 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="UhvXnIU1" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727886AbgGYRgj (ORCPT ); Sat, 25 Jul 2020 13:36:39 -0400 Received: from us-smtp-1.mimecast.com ([205.139.110.61]:56922 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727858AbgGYRgj (ORCPT ); Sat, 25 Jul 2020 13:36:39 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1595698597; 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=B89z7bSsPBlDl23wp5UaTzRFVbTHuOzryOgcGYTtGpc=; b=UhvXnIU1vff9jxxL1kBdKZccTCpmvLl2+pUgkXRsz+wH232dEZOGLfQ9cLemyPNdJggEUq BUtejyuMXEPxvMUdSuMeoy5X5bLFsjWHVFGAktvHrEyUqXhiOjfzegGE/uI73gmUd43Slr nkzKDqLiW160Hr/aPpOeAPWc2iIia5M= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-490-BmOl6uFZMzCjad-UsEKw1g-1; Sat, 25 Jul 2020 13:36:33 -0400 X-MC-Unique: BmOl6uFZMzCjad-UsEKw1g-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id D649E18C63C0; Sat, 25 Jul 2020 17:36:31 +0000 (UTC) Received: from llong.remote.csb (ovpn-112-134.rdu2.redhat.com [10.10.112.134]) by smtp.corp.redhat.com (Postfix) with ESMTP id 462B15FC30; Sat, 25 Jul 2020 17:36:30 +0000 (UTC) Subject: Re: [PATCH v3 5/6] powerpc/pseries: implement paravirt qspinlocks for SPLPAR To: Peter Zijlstra Cc: Will Deacon , Michael Ellerman , Nicholas Piggin , linuxppc-dev@lists.ozlabs.org, Boqun Feng , Ingo Molnar , Anton Blanchard , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, kvm-ppc@vger.kernel.org, linux-arch@vger.kernel.org References: <20200706043540.1563616-1-npiggin@gmail.com> <20200706043540.1563616-6-npiggin@gmail.com> <874kqhvu1v.fsf@mpe.ellerman.id.au> <8265d782-4e50-a9b2-a908-0cb588ffa09c@redhat.com> <20200723140011.GR5523@worktop.programming.kicks-ass.net> <845de183-56f5-2958-3159-faa131d46401@redhat.com> <20200723184759.GS119549@hirez.programming.kicks-ass.net> <20200724081647.GA16642@willie-the-truck> <8532332b-85dd-661b-cf72-81a8ceb70747@redhat.com> <20200725172630.GF10769@hirez.programming.kicks-ass.net> From: Waiman Long Organization: Red Hat Message-ID: <4db0cff6-dabb-2f1b-df66-33ce2082088b@redhat.com> Date: Sat, 25 Jul 2020 13:36:29 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: <20200725172630.GF10769@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 7/25/20 1:26 PM, Peter Zijlstra wrote: > On Fri, Jul 24, 2020 at 03:10:59PM -0400, Waiman Long wrote: >> On 7/24/20 4:16 AM, Will Deacon wrote: >>> On Thu, Jul 23, 2020 at 08:47:59PM +0200, peterz@infradead.org wrote: >>>> On Thu, Jul 23, 2020 at 02:32:36PM -0400, Waiman Long wrote: >>>>> BTW, do you have any comment on my v2 lock holder cpu info qspinlock patch? >>>>> I will have to update the patch to fix the reported 0-day test problem, but >>>>> I want to collect other feedback before sending out v3. >>>> I want to say I hate it all, it adds instructions to a path we spend an >>>> aweful lot of time optimizing without really getting anything back for >>>> it. >>>> >>>> Will, how do you feel about it? >>> I can see it potentially being useful for debugging, but I hate the >>> limitation to 256 CPUs. Even arm64 is hitting that now. >> After thinking more about that, I think we can use all the remaining bits in >> the 16-bit locked_pending. Reserving 1 bit for locked and 1 bit for pending, >> there are 14 bits left. So as long as NR_CPUS < 16k (requirement for 16-bit >> locked_pending), we can put all possible cpu numbers into the lock. We can >> also just use smp_processor_id() without additional percpu data. > That sounds horrific, wouldn't that destroy the whole point of using a > byte for pending? You are right. I realized that later on and had sent a follow-up mail to correct that. >>> Also, you're talking ~1% gains here. I think our collective time would >>> be better spent off reviewing the CNA series and trying to make it more >>> deterministic. >> I thought you guys are not interested in CNA. I do want to get CNA merged, >> if possible. Let review the current version again and see if there are ways >> we can further improve it. > It's not a lack of interrest. We were struggling with the fairness > issues and the complexity of the thing. I forgot the current state of > matters, but at one point UNLOCK was O(n) in waiters, which is, of > course, 'unfortunate'. > > I'll have to look up whatever notes remain, but the basic idea of > keeping remote nodes on a secondary list is obviously breaking all sorts > of fairness. After that they pile on a bunch of hacks to fix the worst > of them, but it feels exactly like that, a bunch of hacks. > > One of the things I suppose we ought to do is see if some of the ideas > of phase-fair locks can be applied to this. That could be a possible solution to ensure better fairness. > > That coupled with a chronic lack of time for anything :-( > That is always true and I feel this way too:-) Cheers, Longman