From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wj0-f200.google.com (mail-wj0-f200.google.com [209.85.210.200]) by kanga.kvack.org (Postfix) with ESMTP id 906C06B0069 for ; Thu, 1 Dec 2016 11:59:23 -0500 (EST) Received: by mail-wj0-f200.google.com with SMTP id xy5so40108806wjc.0 for ; Thu, 01 Dec 2016 08:59:23 -0800 (PST) Received: from mx0a-001b2d01.pphosted.com (mx0b-001b2d01.pphosted.com. [148.163.158.5]) by mx.google.com with ESMTPS id l10si949715wjr.92.2016.12.01.08.59.21 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 01 Dec 2016 08:59:22 -0800 (PST) Received: from pps.filterd (m0098413.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.17/8.16.0.17) with SMTP id uB1GsZ8L068800 for ; Thu, 1 Dec 2016 11:59:21 -0500 Received: from e36.co.us.ibm.com (e36.co.us.ibm.com [32.97.110.154]) by mx0b-001b2d01.pphosted.com with ESMTP id 272kqurrhx-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Thu, 01 Dec 2016 11:59:20 -0500 Received: from localhost by e36.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 1 Dec 2016 09:59:20 -0700 Date: Thu, 1 Dec 2016 08:59:18 -0800 From: "Paul E. McKenney" Subject: Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and `mem_cgroup_shrink_node` Reply-To: paulmck@linux.vnet.ibm.com References: <20161130115320.GO3924@linux.vnet.ibm.com> <20161130131910.GF18432@dhcp22.suse.cz> <20161130142955.GS3924@linux.vnet.ibm.com> <20161130163820.GQ3092@twins.programming.kicks-ass.net> <20161130170557.GK18432@dhcp22.suse.cz> <20161130175015.GR3092@twins.programming.kicks-ass.net> <20161130194019.GF3924@linux.vnet.ibm.com> <20161201053035.GC3092@twins.programming.kicks-ass.net> <20161201124024.GB3924@linux.vnet.ibm.com> <20161201163614.GL3092@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161201163614.GL3092@twins.programming.kicks-ass.net> Message-Id: <20161201165918.GG3924@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Peter Zijlstra Cc: Michal Hocko , Donald Buczek , Paul Menzel , dvteam@molgen.mpg.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Josh Triplett On Thu, Dec 01, 2016 at 05:36:14PM +0100, Peter Zijlstra wrote: > On Thu, Dec 01, 2016 at 04:40:24AM -0800, Paul E. McKenney wrote: > > On Thu, Dec 01, 2016 at 06:30:35AM +0100, Peter Zijlstra wrote: > > > > Sure, we all dislike IPIs, but I'm thinking this half-way point is > > > sensible, no point in issuing user visible annoyance if indeed we can > > > prod things back to life, no? > > > > > > Only if we utterly fail to make it respond should we bug the user with > > > our failure.. > > > > Sold! ;-) > > > > I will put together a patch later today. > > > > My intent is to hold off on the "upgrade cond_resched()" patch, one > > step at a time. Longer term, I do very much like the idea of having > > cond_resched() do both scheduling and RCU quiescent states, assuming > > that this avoids performance pitfalls. > > Well, with the above change cond_resched() is already sufficient, no? Maybe. Right now, cond_resched_rcu_qs() gets a quiescent state to the RCU core in less than one jiffy, with my other change, this becomes a handful of jiffies depending on HZ and NR_CPUS. I expect this increase to a handful of jiffies to be a non-event. After my upcoming patch, cond_resched() will get a quiescent state to the RCU core in about ten seconds. While I am am not all that nervous about the increase from less than a jiffy to a handful of jiffies, increasing to ten seconds via cond_resched() does make me quite nervous. Past experience indicates that someone's kernel will likely be fatally inconvenienced by this magnitude of change. Or am I misunderstanding what you are proposing? > In fact, by doing the IPI thing we get the entire cond_resched*() > family, and we could add the should_resched() guard to > cond_resched_rcu(). So that cond_resched_rcu_qs() looks something like this, in order to avoid the function call in the case where the scheduler has nothing to do? #define cond_resched_rcu_qs() \ do { \ if (!should_resched(current) || !cond_resched()) \ rcu_note_voluntary_context_switch(current); \ } while (0) Thanx, Paul -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org