From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751943AbbJLMRB (ORCPT ); Mon, 12 Oct 2015 08:17:01 -0400 Received: from casper.infradead.org ([85.118.1.10]:49541 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751173AbbJLMQ7 (ORCPT ); Mon, 12 Oct 2015 08:16:59 -0400 Date: Mon, 12 Oct 2015 14:16:57 +0200 From: Peter Zijlstra To: Oleg Nesterov Cc: heiko.carstens@de.ibm.com, Tejun Heo , Ingo Molnar , Rik van Riel , Thomas Gleixner , Vitaly Kuznetsov , linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/3] sched: select_task_rq() should check cpu_active() like select_fallback_rq() Message-ID: <20151012121657.GP3816@twins.programming.kicks-ass.net> References: <20151010185255.GA24075@redhat.com> <20151010185309.GA24089@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151010185309.GA24089@redhat.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Oct 10, 2015 at 08:53:09PM +0200, Oleg Nesterov wrote: > I do not understand the cpu_active() check in select_fallback_rq(). > x86 doesn't need it, and the recent commit dd9d3843755d "sched: Fix > cpu_active_mask/cpu_online_mask race" documents the fact that on any > architecture we can ignore !active starting from CPU_ONLINE stage. > > But any possible reason why do we need this check in "fallback" must > equally apply to select_task_rq(). So the reason, from vague memory, is that we want to allow per-cpu threads to start/stop before/after active. active 'should' really only govern load-balancer bits or so.