Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762894AbdDSMJF (ORCPT ); Wed, 19 Apr 2017 08:09:05 -0400 Received: from merlin.infradead.org ([205.233.59.134]:54640 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1762861AbdDSMJD (ORCPT ); Wed, 19 Apr 2017 08:09:03 -0400 Date: Wed, 19 Apr 2017 14:08:47 +0200 From: Peter Zijlstra To: Christian Borntraeger Cc: "Paul E. McKenney" , 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, marc.zyngier@arm.com Subject: Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12 Message-ID: <20170419120847.GB3029@worktop.programming.kicks-ass.net> References: <20170412165441.GA17149@linux.vnet.ibm.com> <20170417232714.GA19013@linux.vnet.ibm.com> <20170419112845.3rt3zlyzuzert647@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 985 Lines: 21 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.