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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 DB930C433DB for ; Thu, 18 Feb 2021 14:37:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8DF5564EAD for ; Thu, 18 Feb 2021 14:37:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230113AbhBROgj convert rfc822-to-8bit (ORCPT ); Thu, 18 Feb 2021 09:36:39 -0500 Received: from foss.arm.com ([217.140.110.172]:50768 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232923AbhBRMmS (ORCPT ); Thu, 18 Feb 2021 07:42:18 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 3502B1FB; Thu, 18 Feb 2021 04:41:05 -0800 (PST) Received: from e113632-lin (e113632-lin.cambridge.arm.com [10.1.194.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 1A92E3F73D; Thu, 18 Feb 2021 04:41:02 -0800 (PST) From: Valentin Schneider To: "Song Bao Hua \(Barry Song\)" , Peter Zijlstra Cc: "vincent.guittot\@linaro.org" , "mgorman\@suse.de" , "mingo\@kernel.org" , "dietmar.eggemann\@arm.com" , "morten.rasmussen\@arm.com" , "linux-kernel\@vger.kernel.org" , "linuxarm\@openeuler.org" , "xuwei \(O\)" , "Liguozhu \(Kenneth\)" , "tiantao \(H\)" , wanghuiqiang , "Zengtao \(B\)" , Jonathan Cameron , "guodong.xu\@linaro.org" , Meelis Roos Subject: RE: [Linuxarm] Re: [PATCH v2] sched/topology: fix the issue groups don't span domain->span for NUMA diameter > 2 In-Reply-To: References: <20210203111201.20720-1-song.bao.hua@hisilicon.com> <4bdaa3e1a54f445fa8e629ea392e7bce@hisilicon.com> User-Agent: Notmuch/0.21 (http://notmuchmail.org) Emacs/26.3 (x86_64-pc-linux-gnu) Date: Thu, 18 Feb 2021 12:40:53 +0000 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Barry, On 18/02/21 09:17, Song Bao Hua (Barry Song) wrote: > Hi Valentin, > > I understand Peter's concern is that the local group has different > size with remote groups. Is this patch resolving Peter's concern? > To me, it seems not :-) > If you remove the '&& i != cpu' in build_overlap_sched_groups() you get that, but then you also get some extra warnings :-) Now yes, should_we_balance() only matters for the local group. However I'm somewhat wary of messing with the local groups; for one it means you would have more than one tl now accessing the same sgc->next_update, sgc->{min, max}capacity, sgc->group_imbalance (as Vincent had pointed out). By ensuring only remote (i.e. !local) groups are modified (which is what your patch does), we absolve ourselves of this issue, which is why I prefer this approach ATM. > Though I don’t understand why different group sizes will be harmful > since all groups are calculating avg_load and group_type based on > their own capacities. Thus, for a smaller group, its capacity would > be smaller. > > Is it because a bigger group has relatively less chance to pull, so > load balancing will be completed more slowly while small groups have > high load? > Peter's point is that, if at a given tl you have groups that look like g0: 0-4, g1: 5-6, g2: 7-8 Then g0 is half as likely to pull tasks with load_balance() than g1 or g2 (due to the group size vs should_we_balance()) However, I suppose one "trick" to be aware of here is that since your patch *doesn't* change the local group, we do have e.g. on CPU0: [ 0.374840] domain-2: span=0-5 level=NUMA [ 0.375054] groups: 0:{ span=0-3 cap=4003 }, 4:{ span=4-5 cap=1988 } *but* on CPU4 we get: [ 0.387019] domain-2: span=0-1,4-7 level=NUMA [ 0.387211] groups: 4:{ span=4-7 cap=3984 }, 0:{ span=0-1 cap=2013 } IOW, at a given tl, all *local* groups have /roughly/ the same size and thus similar pull probability (it took me writing this mail to see it that way). So perhaps this is all fine already?