Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933484AbcLSWpQ (ORCPT ); Mon, 19 Dec 2016 17:45:16 -0500 Received: from mail.kernel.org ([198.145.29.136]:41624 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932659AbcLSWpO (ORCPT ); Mon, 19 Dec 2016 17:45:14 -0500 MIME-Version: 1.0 In-Reply-To: <20161219224014.GA28238@var.home> References: <20161219224014.GA28238@var.home> From: Paul Turner Date: Mon, 19 Dec 2016 14:44:38 -0800 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics To: Samuel Thibault , LKML , Peter Zijlstra , Thomas Gleixner , Ingo Molnar Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1586 Lines: 42 On Mon, Dec 19, 2016 at 2:40 PM, Samuel Thibault wrote: > 2159197d6677 ("sched/core: Enable increased load resolution on 64-bit kernels") > > exposed yet another miscalculation in calc_cfs_shares: MIN_SHARES is unscaled, > and must thus be scaled before being manipulated against "shares" amounts. It's actually intentional that MIN_SHARES is un-scaled here, this is necessary to support the goal of sub-partitioning groups with small shares. E.g. A group with shares=2 and 5 threads will internally provide 2048 units of weight for the load-balancer to account for their distribution. > > Signed-off-by: Samuel Thibault > Cc: Peter Zijlstra > Cc: Thomas Gleixner > Cc: Ingo Molnar > Cc: stable@vger.kernel.org > Fixes: 2159197d6677 ("sched/core: Enable increased load resolution on 64-bit kernels") > > --- > This should be backported to 4.7 and 4.8 to fix scheduling priorities > miscalculations > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 6559d19..be84f72 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -2657,8 +2657,8 @@ static long calc_cfs_shares(struct cfs_rq *cfs_rq, struct task_group *tg) > if (tg_weight) > shares /= tg_weight; > > - if (shares < MIN_SHARES) > - shares = MIN_SHARES; > + if (shares < scale_load(MIN_SHARES)) > + shares = scale_load(MIN_SHARES); > if (shares > tg->shares) > shares = tg->shares; >