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=-6.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_NEOMUTT autolearn=ham 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 AA330C43381 for ; Tue, 19 Mar 2019 12:33:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7D7C120857 for ; Tue, 19 Mar 2019 12:33:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727143AbfCSMd5 (ORCPT ); Tue, 19 Mar 2019 08:33:57 -0400 Received: from outbound-smtp25.blacknight.com ([81.17.249.193]:51512 "EHLO outbound-smtp25.blacknight.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726002AbfCSMd5 (ORCPT ); Tue, 19 Mar 2019 08:33:57 -0400 Received: from mail.blacknight.com (pemlinmail05.blacknight.ie [81.17.254.26]) by outbound-smtp25.blacknight.com (Postfix) with ESMTPS id B78F9B8700 for ; Tue, 19 Mar 2019 12:33:55 +0000 (GMT) Received: (qmail 19748 invoked from network); 19 Mar 2019 12:33:55 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[213.151.95.130]) by 81.17.254.9 with ESMTPSA (DHE-RSA-AES256-SHA encrypted, authenticated); 19 Mar 2019 12:33:55 -0000 Date: Tue, 19 Mar 2019 12:33:54 +0000 From: Mel Gorman To: Peter Zijlstra Cc: Ingo Molnar , linux-kernel@vger.kernel.org Subject: Re: [PATCH] sched: Do not re-read h_load_next during hierarchical load calculation Message-ID: <20190319123354.qnsazogerjqa2dmv@techsingularity.net> References: <20190319091709.lqrtbn76sjx73hnv@techsingularity.net> <20190319120609.GD5996@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20190319120609.GD5996@hirez.programming.kicks-ass.net> User-Agent: NeoMutt/20170912 (1.9.0) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 19, 2019 at 01:06:09PM +0100, Peter Zijlstra wrote: > > --- > > kernel/sched/fair.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index 310d0637fe4b..34aeb40e69d2 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -7726,7 +7726,7 @@ static void update_cfs_rq_h_load(struct cfs_rq *cfs_rq) > > cfs_rq->last_h_load_update = now; > > } > > > > - while ((se = cfs_rq->h_load_next) != NULL) { > > + while ((se = READ_ONCE(cfs_rq->h_load_next)) != NULL) { > > load = cfs_rq->h_load; > > load = div64_ul(load * se->avg.load_avg, > > cfs_rq_load_avg(cfs_rq) + 1); > > Where there is a READ_ONCE there should also be a corresponding > WRITE_ONCE(). Otherwise the compiler can still screw us over by doing > store-tearing. > > So something like the below. But looking at this, we probably also want > ONCE treatment on cfs_rq->h_load itself, but that's another patch. > > And I think we can do something with cfs_rq->last_h_load_update. > Ok, I hadn't taken into account the possibility of store tearing but you're right, it is a possibility depending on the architecture and your patch is better. While there are some potential issues around h_load, I had treated it as data that is approximate and only considered the potential NULL dereference to be relevant. I'll send a v2 shortly.