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 80A87C433E0 for ; Sat, 25 Jul 2020 03:03:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 573BE206EB for ; Sat, 25 Jul 2020 03:03:10 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Anfbl2kc" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726782AbgGYDDI (ORCPT ); Fri, 24 Jul 2020 23:03:08 -0400 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:55504 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726572AbgGYDDH (ORCPT ); Fri, 24 Jul 2020 23:03:07 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1595646186; 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=dJKlmgtXl7uNqK+gi01GLPol8dXOhIFF+vRj8dKFHJ4=; b=Anfbl2kcsBmd0PI36qol9zAkRgk0jUE4SYT3TPGKkKjzvAS3che2+YFxVTPSqRXDyfgpzm LmpNe/cBem1mtkTk9js6lFbJ4UVjgW3QeWTr2joXPbTaN1ezEPrLtj+xi4sTGzyCy+lZ2W jk5KGjDzM1hkF1Z/u+KA3PY92MHdlsI= 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-388-taJbuqtoOr-9IUyajzXA8A-1; Fri, 24 Jul 2020 23:03:02 -0400 X-MC-Unique: taJbuqtoOr-9IUyajzXA8A-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id AA6B958; Sat, 25 Jul 2020 03:03:00 +0000 (UTC) Received: from llong.remote.csb (ovpn-117-203.rdu2.redhat.com [10.10.117.203]) by smtp.corp.redhat.com (Postfix) with ESMTP id B17D18FA2A; Sat, 25 Jul 2020 03:02:58 +0000 (UTC) Subject: Re: [PATCH v3 5/6] powerpc/pseries: implement paravirt qspinlocks for SPLPAR From: Waiman Long To: Will Deacon , peterz@infradead.org Cc: 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> Organization: Red Hat Message-ID: Date: Fri, 24 Jul 2020 23:02:58 -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: <8532332b-85dd-661b-cf72-81a8ceb70747@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 7/24/20 3:10 PM, 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. Sorry, that doesn't work. The extra bits in the pending byte won't get cleared on unlock. That will have noticeable performance impact. Clearing the pending byte on unlock will cause other performance problem. So I guess we will have to limit the cpu number in the locked byte. Regards, Longman