Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755500AbYGaNJS (ORCPT ); Thu, 31 Jul 2008 09:09:18 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751177AbYGaNJF (ORCPT ); Thu, 31 Jul 2008 09:09:05 -0400 Received: from x346.tv-sign.ru ([89.108.83.215]:60595 "EHLO mail.screens.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751085AbYGaNJE (ORCPT ); Thu, 31 Jul 2008 09:09:04 -0400 Date: Thu, 31 Jul 2008 17:12:26 +0400 From: Oleg Nesterov To: Dmitry Adamushko Cc: Linus Torvalds , Andrew Morton , Ingo Molnar , Roland McGrath , linux-kernel@vger.kernel.org Subject: Re: [PATCH 3/3] wait_task_inactive: don't use the dummy version when !SMP && PREEMPT Message-ID: <20080731131226.GB76@tv-sign.ru> References: <20080730170949.GA18682@tv-sign.ru> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.11 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2296 Lines: 60 On 07/30, Dmitry Adamushko wrote: > > 2008/7/30 Linus Torvalds : > > > > Oh, and shouldn't it do a "yield()" instead of a cpu_relax() on UP? > > This part could have been skipped for UP. task_running(rq, p) just > can't give 'true' for UP (otherwise it's a bug). The only relevant > part is "on_rq = p->se.on_rq". That was my understanding, thanks for your confirmation. > (if [1], then I think a separate function for PREEMPT would look > better, i.e. without parts with task_running()) in that case we can also eliminate task_rq_lock() afaics. > e.g. consider this code from kthread_bind(): > > /* Must have done schedule() in kthread() before we set_task_cpu */ > wait_task_inactive(k, 0); > > set_task_cpu(k, cpu); > k->cpus_allowed = cpumask_of_cpu(cpu); > k->rt.nr_cpus_allowed = 1; > k->flags |= PF_THREAD_BOUND; > > set_task_cpu(k, cpu) is safe _only_ if 'k' is not on the run-queue > (and can't be placed onto it behind our back -- heh, a bit subtle). > > Now, for !SMP + PREEMPT it's not a case. set_task_cpu() may be called > while 'k' is still on the run-queue (more precisely, preempted in > kthread() between complete(&create->started); and schedule();). > > Yes, set_task_cpu() is a "nop" for UP so that's ok in this particular > case. But let's suppose, another use-case would be introduced with > 'false' assumptions causing troubles for !SMP. Completely agreed. Currently the only user which can suffer on UP && PREEMPT is task_current_syscall(). As Roland suggested we can fix it if we change the !SMP version to return ->nivcsw + ->nvcsw. But personally I don't like the fact that we will have the subltle difference in behaviour depending on CONFIG_SMP. We have to wait for .on_rq == 0 on SMP, imho it is better to do the same on UP even if none of the current callers needs this. That said, I think this problem is minor and I don't (and can't) have the strong opinion on how to fix it. I'd better wait for the authoritative verdict. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/