Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id A7B26C678D4 for ; Thu, 2 Mar 2023 15:03:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229985AbjCBPD5 (ORCPT ); Thu, 2 Mar 2023 10:03:57 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33690 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229520AbjCBPDy (ORCPT ); Thu, 2 Mar 2023 10:03:54 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 39DC8158B5 for ; Thu, 2 Mar 2023 07:03:02 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1677769381; 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: in-reply-to:in-reply-to:references:references; bh=LVBHeaUDd3fMfdryiaZTF5F5Bv975tBnlPCSsjv5Pzs=; b=AE3JvrTtA6o+L/tN3KFTT7eXfnB1+ATjT8JoPnQF/UEUx5kH7l9Wi/LxxClkslGIE7Dcji 17O5wNWXVdCbYe+QbgsdFAgRVTCS9DiHW5vpE9rFOOV6ls2lrIy/PiNUMOZZN5LMgDIzK8 2qMar/wP5UlkNPSjmbSCPYteW4jRSW4= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-203-1PGCLXHvPm-glmSZu0fXJA-1; Thu, 02 Mar 2023 09:48:48 -0500 X-MC-Unique: 1PGCLXHvPm-glmSZu0fXJA-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 8117485A5B1; Thu, 2 Mar 2023 14:48:47 +0000 (UTC) Received: from tpad.localdomain (ovpn-112-2.gru2.redhat.com [10.97.112.2]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 4C6BF492C3E; Thu, 2 Mar 2023 14:48:47 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id D78F7403CC770; Thu, 2 Mar 2023 11:32:06 -0300 (-03) Date: Thu, 2 Mar 2023 11:32:06 -0300 From: Marcelo Tosatti To: David Hildenbrand Cc: Christoph Lameter , Aaron Tomlin , Frederic Weisbecker , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v2 02/11] this_cpu_cmpxchg: ARM64: switch this_cpu_cmpxchg to locked, add _local function Message-ID: References: <20230209150150.380060673@redhat.com> <20230209153204.683821550@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.1 on 10.11.54.10 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 02, 2023 at 11:42:57AM +0100, David Hildenbrand wrote: > On 09.02.23 16:01, Marcelo Tosatti wrote: > > Goal is to have vmstat_shepherd to transfer from > > per-CPU counters to global counters remotely. For this, > > an atomic this_cpu_cmpxchg is necessary. > > > > Following the kernel convention for cmpxchg/cmpxchg_local, > > change ARM's this_cpu_cmpxchg_ helpers to be atomic, > > and add this_cpu_cmpxchg_local_ helpers which are not atomic. > > > > Signed-off-by: Marcelo Tosatti > > > > Index: linux-vmstat-remote/arch/arm64/include/asm/percpu.h > > =================================================================== > > --- linux-vmstat-remote.orig/arch/arm64/include/asm/percpu.h > > +++ linux-vmstat-remote/arch/arm64/include/asm/percpu.h > > @@ -232,13 +232,23 @@ PERCPU_RET_OP(add, add, ldadd) > > _pcp_protect_return(xchg_relaxed, pcp, val) > > #define this_cpu_cmpxchg_1(pcp, o, n) \ > > - _pcp_protect_return(cmpxchg_relaxed, pcp, o, n) > > + _pcp_protect_return(cmpxchg, pcp, o, n) > > #define this_cpu_cmpxchg_2(pcp, o, n) \ > > - _pcp_protect_return(cmpxchg_relaxed, pcp, o, n) > > + _pcp_protect_return(cmpxchg, pcp, o, n) > > #define this_cpu_cmpxchg_4(pcp, o, n) \ > > - _pcp_protect_return(cmpxchg_relaxed, pcp, o, n) > > + _pcp_protect_return(cmpxchg, pcp, o, n) > > #define this_cpu_cmpxchg_8(pcp, o, n) \ > > + _pcp_protect_return(cmpxchg, pcp, o, n) > > + > > +#define this_cpu_cmpxchg_local_1(pcp, o, n) \ > > _pcp_protect_return(cmpxchg_relaxed, pcp, o, n) > > +#define this_cpu_cmpxchg_local_2(pcp, o, n) \ > > + _pcp_protect_return(cmpxchg_relaxed, pcp, o, n) > > +#define this_cpu_cmpxchg_local_4(pcp, o, n) \ > > + _pcp_protect_return(cmpxchg_relaxed, pcp, o, n) > > +#define this_cpu_cmpxchg_local_8(pcp, o, n) \ > > + _pcp_protect_return(cmpxchg_relaxed, pcp, o, n) > > + > > Call me confused (not necessarily your fault :) ). > > We have cmpxchg_local, cmpxchg_relaxed and cmpxchg. this_cpu_cmpxchg_local_* > now calls ... *drumroll* ... cmpxchg_relaxed. > IIUC, cmpxchg_local is only guaranteed to be atomic WRO the current CPU > (especially, protection against interrupts when the operation is implemented > using multiple instructions). We do have a generic implementation that > disables/enables interrupts. > > IIUC, cmpxchg_relaxed an atomic update without any memory ordering > guarantees (in contrast to cmpxchg, cmpxchg_acquire, cmpxchg_acquire). We > default to arch_cmpxchg if we don't have arch_cmpxchg_relaxed. arch_cmpxchg > defaults to arch_cmpxchg_local, if not supported. > > > Naturally I wonder: > > (a) Should these new variants be rather called > this_cpu_cmpxchg_relaxed_* ? No: it happens that on ARM-64 cmpxchg_local == cmpxchg_relaxed. See cf10b79a7d88edc689479af989b3a88e9adf07ff. > (b) Should these new variants rather call the "_local" variant? They probably should. But this patchset maintains the current behaviour of this_cpu_cmpxch (for this_cpu_cmpxch_local), which was: #define this_cpu_cmpxchg_1(pcp, o, n) \ - _pcp_protect_return(cmpxchg_relaxed, pcp, o, n) + _pcp_protect_return(cmpxchg, pcp, o, n) #define this_cpu_cmpxchg_2(pcp, o, n) \ - _pcp_protect_return(cmpxchg_relaxed, pcp, o, n) + _pcp_protect_return(cmpxchg, pcp, o, n) #define this_cpu_cmpxchg_4(pcp, o, n) \ - _pcp_protect_return(cmpxchg_relaxed, pcp, o, n) + _pcp_protect_return(cmpxchg, pcp, o, n) #define this_cpu_cmpxchg_8(pcp, o, n) \ + _pcp_protect_return(cmpxchg, pcp, o, n) Thanks.