From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751156AbdAQMMe (ORCPT ); Tue, 17 Jan 2017 07:12:34 -0500 Received: from mx2.suse.de ([195.135.220.15]:54654 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750826AbdAQMMb (ORCPT ); Tue, 17 Jan 2017 07:12:31 -0500 Date: Tue, 17 Jan 2017 13:11:46 +0100 From: Michal Hocko To: "Paul E. McKenney" Cc: Peter Zijlstra , linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com Subject: Re: [PATCH tip/core/rcu 11/20] sched,rcu: Make cond_resched() provide RCU quiescent state Message-ID: <20170117121146.GK19699@dhcp22.suse.cz> References: <20170114091255.GA20854@linux.vnet.ibm.com> <1484385201-22227-11-git-send-email-paulmck@linux.vnet.ibm.com> <20170116171130.GC6500@twins.programming.kicks-ass.net> <20170117005403.GW5238@linux.vnet.ibm.com> <20170117105141.GI19699@dhcp22.suse.cz> <20170117120513.GB5238@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170117120513.GB5238@linux.vnet.ibm.com> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 17-01-17 04:05:13, Paul E. McKenney wrote: > On Tue, Jan 17, 2017 at 11:51:41AM +0100, Michal Hocko wrote: > > On Mon 16-01-17 16:54:03, Paul E. McKenney wrote: > > > On Mon, Jan 16, 2017 at 06:11:30PM +0100, Peter Zijlstra wrote: > > > > On Sat, Jan 14, 2017 at 01:13:12AM -0800, Paul E. McKenney wrote: > > > > > There is some confusion as to which of cond_resched() or > > > > > cond_resched_rcu_qs() should be added to long in-kernel loops. > > > > > This commit therefore eliminates the decision by adding RCU > > > > > quiescent states to cond_resched(). > > > > > > > > Which would make: rcu_read_lock(); cond_resched(); rcu_read_unlock(); > > > > invalid under preemptible RCU. Is it already? > > > > > > In theory, yes. In practice, I just tested it with preemption and > > > lockdep enabled, and it didn't complain. If further testing finds > > > complaints, we can either fix those uses (preferred) or revert > > > this patch. > > > > > > > > Warning: This is a prototype. For example, it does not correctly > > > > > handle Tasks RCU. Which is OK for the moment, given that no one > > > > > actually uses Tasks RCU yet. > > > > > > > > > --- a/kernel/sched/core.c > > > > > +++ b/kernel/sched/core.c > > > > > @@ -4907,6 +4907,7 @@ int __sched _cond_resched(void) > > > > > preempt_schedule_common(); > > > > > return 1; > > > > > } > > > > > + rcu_all_qs(); > > > > > return 0; > > > > > } > > > > > > > > Still not a real fan of this, it does make cond_resched() touch a bunch > > > > more cachelines, also, I suppose that if we're going to do this, we > > > > should make __cond_resched_lock() and __cond_resched_softirq() act > > > > similarly. > > > > > > Michal (now CCed) argues that having to distinguish between cond_resched() > > > and cond_resched_rcu_qs() is overly burdensome. Michal? > > > > Yes, it is really not clear which one is meant to be in which context. I > > really do not see which cond_resched should be turned intto > > cond_resched_rcu_qs. > > > > > Any thoughts on how we might remove this burden without the additional > > > cache misses? I will take another look as well to see what could make > > > it lower cost. There are probably ways... Would it make sense to > > > have RCU maintain a need-rcu_all_qs() flage in the same cacheline as > > > the __preempt_count? Perhaps throttling the writes to this flag from > > > the RCU grace-period kthreads to once per 100 milliseconds or so? > > > > Can the stall detector simply request rescheduling when it gets > > dangerously close to the timeout? > > It is quite possible that half of the stall timeout would be a better > choice than my 100 milliseconds, but either way, there would be need > for a flag or some such. E.g. set_tsk_need_resched() on the task currently running on a cpu which is preventing the rcu grace period for too long? That would only require change to the stall detector and the cond_resched could be left alone completely. -- Michal Hocko SUSE Labs