From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759964AbcILPBe (ORCPT ); Mon, 12 Sep 2016 11:01:34 -0400 Received: from merlin.infradead.org ([205.233.59.134]:42432 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758209AbcILPB2 (ORCPT ); Mon, 12 Sep 2016 11:01:28 -0400 Date: Mon, 12 Sep 2016 17:01:11 +0200 From: Peter Zijlstra To: Oleg Nesterov Cc: Cheng Chao , mingo@kernel.org, tj@kernel.org, akpm@linux-foundation.org, chris@chris-wilson.co.uk, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful works for CONFIG_PREEMPT_NONE Message-ID: <20160912150111.GC10121@twins.programming.kicks-ass.net> References: <1473408834-6122-1-git-send-email-chengchao@kedacom.com> <1473497532-5440-1-git-send-email-cs.os.kernel@gmail.com> <20160912113727.GQ10153@twins.programming.kicks-ass.net> <20160912114108.GP10168@twins.programming.kicks-ass.net> <20160912130537.GB7984@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160912130537.GB7984@redhat.com> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 12, 2016 at 03:05:38PM +0200, Oleg Nesterov wrote: > But this leads to the question which I wanted to ask many times. > > Why cond_resched() is not NOP if CONFIG_PREEMPT=y ? Dunno, nobody bothered to do it? We should keep the might_sleep() of course, but the preemption check is pointless. > Perhaps we have some users like, just for example, > > preempt_enable_no_resched(); > cond_resched(); > > which actually want the should_resched() check even if CONFIG_PREEMPT, > but most callers do not? I would hope not, the few preempt_enable_no_resched() users _should_ have an actual schedule() call in the _very_ near future.