Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S936714AbdDSO7A (ORCPT ); Wed, 19 Apr 2017 10:59:00 -0400 Received: from relay7-d.mail.gandi.net ([217.70.183.200]:57275 "EHLO relay7-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S936700AbdDSO65 (ORCPT ); Wed, 19 Apr 2017 10:58:57 -0400 Date: Wed, 19 Apr 2017 07:58:44 -0700 From: Josh Triplett To: "Paul E. McKenney" Cc: Peter Zijlstra , Christian Borntraeger , linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, 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, marc.zyngier@arm.com Subject: Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 Message-ID: <20170419145844.GA17080@cloud> References: <20170412165441.GA17149@linux.vnet.ibm.com> <20170417232714.GA19013@linux.vnet.ibm.com> <20170419112845.3rt3zlyzuzert647@hirez.programming.kicks-ass.net> <20170419120847.GB3029@worktop.programming.kicks-ass.net> <20170419144730.GA20845@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170419144730.GA20845@linux.vnet.ibm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3451 Lines: 72 On Wed, Apr 19, 2017 at 07:47:30AM -0700, Paul E. McKenney wrote: > On Wed, Apr 19, 2017 at 02:08:47PM +0200, Peter Zijlstra wrote: > > On Wed, Apr 19, 2017 at 01:48:08PM +0200, Christian Borntraeger wrote: > > > On 04/19/2017 01:28 PM, Peter Zijlstra wrote: > > > > > > > > So the thing Maz complained about is because KVM assumes > > > > synchronize_srcu() is 'free' when there is no srcu_read_lock() activity. > > > > This series 'breaks' that. > > > > > > Why is such a behaviour change not mentioned in the cover letter? > > > I could not find anything in the patch descriptions that would > > > indicate a slowdown. How much slower did it get? > > > > > > But indeed, there are several places at KVM startup which have been > > > reworked to srcu since normal rcu was too slow for several usecases. > > > (Mostly registering devices and related data structures at startup, > > > basically the qemu/kvm coldplug interaction) > > > > I suspect Paul is not considering this a 'normal' RCU feature, and > > therefore didn't think about changing this. > > > > I know I was fairly surprised by this requirement when I ran into it; > > and only accidentally remembered it now that maz complained. > > Indeed -- the natural thing to have done back when KVM's scalability was > first being worked on would have been to simply change synchronize_rcu() > to synchronize_rcu_expedited(). However, at that time, these things > did try_stop_cpus() and the like, which was really bad for latency. > Moving to SRCU avoided this problem. Of course, now that KVM uses > SRCU, why change unless there is a problem? Besides, I vaguely recall > some KVM cases where srcu_read_lock() is used from CPUs that look to > be idle or offline from RCU's perspective, and that sort of thing only > works for SRCU. > > Which reminds me... > > The RCU expedited primitives have been completely rewritten since then, > and no longer use try_stop_cpus(), no longer disturb idle CPUs, and no > longer disturb nohz_full CPUs running in userspace. In addition, there > is the rcupdate.rcu_normal kernel boot paramter for those who want to > completely avoid RCU expedited primitives. > > So it seems to me to be time for the patch below. Thoughts? > > Thanx, Paul > > ------------------------------------------------------------------------ > > commit 333d383fad42b4bdef3d27d91e940a6eafed3f91 > Author: Paul E. McKenney > Date: Wed Apr 19 07:37:45 2017 -0700 > > checkpatch: Remove checks for expedited grace periods > > There was a time when the expedited grace-period primitives > (synchronize_rcu_expedited(), synchronize_rcu_bh_expedited(), and > synchronize_sched_expedited()) used rather antisocial kernel > facilities like try_stop_cpus(). However, they have since been > housebroken to use only single-CPU IPIs, and typically cause less > disturbance than a scheduling-clock interrupt. Furthermore, this > disturbance can be eliminated entirely using NO_HZ_FULL on the > one hand or the rcupdate.rcu_normal boot parameter on the other. > > This commit therefore removes checkpatch's complaints about use > of the expedited RCU primitives. > > Signed-off-by: Paul E. McKenney Reviewed-by: Josh Triplett Still something to hesitate a bit before using, but not something checkpatch should warn about.