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 1A77DC0032E for ; Sat, 28 Oct 2023 15:03:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229492AbjJ1PDa (ORCPT ); Sat, 28 Oct 2023 11:03:30 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:32922 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229446AbjJ1PDa (ORCPT ); Sat, 28 Oct 2023 11:03:30 -0400 Received: from mail2-relais-roc.national.inria.fr (mail2-relais-roc.national.inria.fr [192.134.164.83]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D7D869D for ; Sat, 28 Oct 2023 08:03:26 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=inria.fr; s=dc; h=date:from:to:cc:subject:in-reply-to:message-id: references:mime-version; bh=JCgC8iEfqeVG5SmoD+Z1qjOZtlgkC88g0qaak8aF7AM=; b=HwvWRDA7Sgno0G1lAYJ4S8dFlOw/uCUwDTPVjlc61GeP9Fl7fV96iEfB u9SBd+tk6u+pE0SatqvwzY4NmVwZL84fjhx5soQ8lnE2clzkgBbFCh3Df HXpW/QvsvygxkReqhxcvSArAn14eWPH+SFjSgJWvec3LDNKb7Bj6FMSMF w=; Authentication-Results: mail2-relais-roc.national.inria.fr; dkim=none (message not signed) header.i=none; spf=SoftFail smtp.mailfrom=julia.lawall@inria.fr; dmarc=fail (p=none dis=none) d=inria.fr X-IronPort-AV: E=Sophos;i="6.03,259,1694728800"; d="scan'208";a="133701260" Received: from 231.85.89.92.rev.sfr.net (HELO hadrien) ([92.89.85.231]) by mail2-relais-roc.national.inria.fr with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 28 Oct 2023 17:03:23 +0200 Date: Sat, 28 Oct 2023 17:03:23 +0200 (CEST) From: Julia Lawall X-X-Sender: jll@hadrien To: Chen Yu cc: Keisuke Nishimura , Ingo Molnar , Peter Zijlstra , Vincent Guittot , linux-kernel@vger.kernel.org, Dietmar Eggemann , Mel Gorman , Valentin Schneider , Ricardo Neri , Tim Chen Subject: Re: [PATCH] sched/fair: Fix the decision for load balance In-Reply-To: Message-ID: References: <20231027171742.1426070-1-keisuke.nishimura@inria.fr> User-Agent: Alpine 2.22 (DEB 394 2020-01-19) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 28 Oct 2023, Chen Yu wrote: > On 2023-10-28 at 08:37:59 +0200, Julia Lawall wrote: > > > > > > On Sat, 28 Oct 2023, Chen Yu wrote: > > > > > On 2023-10-27 at 19:17:43 +0200, Keisuke Nishimura wrote: > > > > should_we_balance is called for the decision to do load-balancing. > > > > When sched ticks invoke this function, only one CPU should return > > > > true. However, in the current code, two CPUs can return true. The > > > > following situation, where b means busy and i means idle, is an > > > > example because CPU 0 and CPU 2 return true. > > > > > > > > [0, 1] [2, 3] > > > > b b i b > > > > > > > > This fix checks if there exists an idle CPU with busy sibling(s) > > > > after looking for a CPU on an idle core. If some idle CPUs with busy > > > > siblings are found, just the first one should do load-balancing. > > > > > > > > Fixes: b1bfeab9b002 ("sched/fair: Consider the idle state of the whole core for load balance") > > > > Signed-off-by: Keisuke Nishimura > > > > --- > > > > kernel/sched/fair.c | 5 +++-- > > > > 1 file changed, 3 insertions(+), 2 deletions(-) > > > > > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > > > index 2048138ce54b..eff0316d6c7d 100644 > > > > --- a/kernel/sched/fair.c > > > > +++ b/kernel/sched/fair.c > > > > @@ -11083,8 +11083,9 @@ static int should_we_balance(struct lb_env *env) > > > > return cpu == env->dst_cpu; > > > > } > > > > > > > > - if (idle_smt == env->dst_cpu) > > > > - return true; > > > > + /* Is there an idle CPU with busy siblings? */ > > > > + if (idle_smt != -1) > > > > + return idle_smt == env->dst_cpu; > > > > > > > > /* Are we the first CPU of this group ? */ > > > > return group_balance_cpu(sg) == env->dst_cpu; > > > > > > Looks reasonable to me, if there is other idle SMT(from half-busy core) > > > in the system, we should leverage that SMT to do the periodic lb. > > > Per my understanding, > > > > That's not the goal of this patch. The goal of this patch is to avoid > > doing return group_balance_cpu(sg) == env->dst_cpu; > > Yes, I mean, without this patch, we could incorrectly choose the current > non idle CPU rather than that idle SMT, but actually we should let that > idle SMT to do the idle lb. OK, agreed. Thanks for the feedback! julia > > thanks, > Chenyu > > > when a half-busy core > > has been identified that is different from env->dst_cpu. > > > > julia > > > > > > > > Reviewed-by: Chen Yu > > > > > > thanks, > > > Chenyu > > > >