From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755775AbdESOXj (ORCPT ); Fri, 19 May 2017 10:23:39 -0400 Received: from mail.kernel.org ([198.145.29.99]:60060 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755632AbdESOXe (ORCPT ); Fri, 19 May 2017 10:23:34 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DE2992395D Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=goodmis.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=rostedt@goodmis.org Date: Fri, 19 May 2017 10:23:31 -0400 From: Steven Rostedt To: "Paul E. McKenney" Cc: Ingo Molnar , linux-kernel@vger.kernel.org, Peter Zijlstra , Thomas Gleixner Subject: Re: Use case for TASKS_RCU Message-ID: <20170519102331.0d5a8536@gandalf.local.home> In-Reply-To: <20170519100421.27298063@gandalf.local.home> References: <20170515182354.GA25440@linux.vnet.ibm.com> <20170516062233.tyz7ze7ilmbkxtjc@gmail.com> <20170516122354.GB3956@linux.vnet.ibm.com> <20170519062331.52dhungzvcsdxdgo@gmail.com> <20170519133550.GD3956@linux.vnet.ibm.com> <20170519100421.27298063@gandalf.local.home> X-Mailer: Claws Mail 3.14.0 (GTK+ 2.24.31; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 19 May 2017 10:04:21 -0400 Steven Rostedt wrote: > On Fri, 19 May 2017 06:35:50 -0700 > "Paul E. McKenney" wrote: > > > Simpler would be better! > > > > However, is it really guaranteed that one SCHED_IDLE thread cannot > > preempt another? If not, then the trampoline-freeing SCHED_IDLE thread > > might preempt some other SCHED_IDLE thread in the middle of a trampoline. > > I am not seeing anything that prevents such preemption, but it is rather > > early local time, so I could easily be missing something. > > > > However, if SCHED_IDLE threads cannot preempt other threads, even other > > SCHED_IDLE threads, then your approach sounds quite promising to me. > > > > Steve, Peter, thoughts? > > SCHED_IDLE is the swapper task. There's one on each CPU, and they don't > migrate. And they only get called when there's no other task running. Peter just "schooled" me on IRC. I stand corrected (and he may respond to this email too). I guess any task can become SCHED_IDLE. But that just makes this an even less likely option for synchronize_rcu_tasks(). -- Steve