From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Subject: Re: [PATCH v3 5/6] powerpc/pseries: implement paravirt qspinlocks for SPLPAR Date: Fri, 24 Jul 2020 15:10:59 -0400 Message-ID: <8532332b-85dd-661b-cf72-81a8ceb70747@redhat.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:53358 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726666AbgGXTLI (ORCPT ); Fri, 24 Jul 2020 15:11:08 -0400 In-Reply-To: <20200724081647.GA16642@willie-the-truck> Content-Language: en-US Sender: linux-arch-owner@vger.kernel.org List-ID: 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 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. > > 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. Cheers, Longman