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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id E1714C6FA8E for ; Thu, 2 Mar 2023 14:51:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229740AbjCBOvt (ORCPT ); Thu, 2 Mar 2023 09:51:49 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40850 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229457AbjCBOvs (ORCPT ); Thu, 2 Mar 2023 09:51:48 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4DB1A58B60 for ; Thu, 2 Mar 2023 06:49:55 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1677768532; 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=MS5VgBYAjiu1GtS1Hz9hknenkB/L3zfoqR0xXE1AVFY=; b=Sbc5xau8yizsgHgn68m3z979xWBAJWgPNQOR6Q0RFOHFo2GKxQvXUj/Ru8YdT7w/H2TIGm IAabrIXhcNwT+QEQyKsaxH543Gd7KkKpnYOSCqb9llMsh17PhjGxDlv/rci3p/bclk8lQ6 u4GxOMPaBlEXzxdXmkJ7lBUrg0JUnyw= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-199-CcJVn-9SM7eNka9q6e28rA-1; Thu, 02 Mar 2023 09:48:48 -0500 X-MC-Unique: CcJVn-9SM7eNka9q6e28rA-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 6A43F2800490; 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 2E2931121315; Thu, 2 Mar 2023 14:48:47 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id 6C139400E71AF; Thu, 2 Mar 2023 10:55:09 -0300 (-03) Date: Thu, 2 Mar 2023 10:55:09 -0300 From: Marcelo Tosatti To: Peter Xu Cc: Christoph Lameter , Aaron Tomlin , Frederic Weisbecker , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v2 09/11] mm/vmstat: use cmpxchg loop in cpu_vm_stats_fold Message-ID: References: <20230209150150.380060673@redhat.com> <20230209153204.873999366@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 10.11.54.3 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 01, 2023 at 05:57:08PM -0500, Peter Xu wrote: > On Thu, Feb 09, 2023 at 12:01:59PM -0300, Marcelo Tosatti wrote: > > /* > > - * Fold the data for an offline cpu into the global array. > > + * Fold the data for a cpu into the global array. > > * There cannot be any access by the offline cpu and therefore > > * synchronization is simplified. > > */ > > @@ -906,8 +906,9 @@ void cpu_vm_stats_fold(int cpu) > > if (pzstats->vm_stat_diff[i]) { > > int v; > > > > - v = pzstats->vm_stat_diff[i]; > > - pzstats->vm_stat_diff[i] = 0; > > + do { > > + v = pzstats->vm_stat_diff[i]; > > + } while (!try_cmpxchg(&pzstats->vm_stat_diff[i], &v, 0)); > > IIUC try_cmpxchg will update "v" already, so I'd assume this'll work the > same: > > while (!try_cmpxchg(&pzstats->vm_stat_diff[i], &v, 0)); > > Then I figured, maybe it's easier to use xchg()? Yes, fixed. > I've no knowledge at all on cpu offline code, so sorry if this will be a > naive question. But from what I understand this should not be touched by > anyone else. Reasons: > > (1) cpu_vm_stats_fold() is only called in page_alloc_cpu_dead(), and the > comment says: > > /* > * Zero the differential counters of the dead processor > * so that the vm statistics are consistent. > * > * This is only okay since the processor is dead and cannot > * race with what we are doing. > */ > cpu_vm_stats_fold(cpu); > > so.. I think that's what it says.. This refers to the use of this_cpu operations being performed by the counter updates. If both the updater and reader use atomic accesses (which is the case after patch 8: "mm/vmstat: switch counter modification to cmpxchg"), and CONFIG_HAVE_CMPXCHG_LOCAL is set, then the comment is stale. Removed it. > (2) If someone can modify the dead cpu's vm_stat_diff, The only context that can modify the cpu's vm_stat_diff are: 1) The CPU itself (increases the counter). 2) cpu_vm_stats_fold (from vmstat_shepherd kernel thread), from x -> 0 only. So you should not be able to increase the counter after this point. I suppose this is what this comment refers to. > what guarantees it > won't be e.g. boosted again right after try_cmpxchg() / xchg() > returns? What to do with the left-overs? If any code runs on the CPU that is being hotunplugged, after cpu_vm_stats_fold (from page_alloc_cpu_dead), then there will be left-overs. But such bugs would exist today as well. Or, if that bug exists, you could replace "for_each_online_cpu" to "for_each_cpu" here: static void vmstat_shepherd(struct work_struct *w) { int cpu; cpus_read_lock(); /* Check processors whose vmstat worker threads have been disabled */ for_each_online_cpu(cpu) {