From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S937694AbdDSPn4 (ORCPT ); Wed, 19 Apr 2017 11:43:56 -0400 Received: from bombadil.infradead.org ([65.50.211.133]:44922 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S937679AbdDSPny (ORCPT ); Wed, 19 Apr 2017 11:43:54 -0400 Date: Wed, 19 Apr 2017 17:43:43 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: 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: <20170419154343.osootv4bcwfppnlc@hirez.programming.kicks-ass.net> References: <20170412165441.GA17149@linux.vnet.ibm.com> <20170417232714.GA19013@linux.vnet.ibm.com> <20170419112845.3rt3zlyzuzert647@hirez.programming.kicks-ass.net> <20170419130245.GJ3956@linux.vnet.ibm.com> <20170419131553.cii62nl2si3lwm7w@hirez.programming.kicks-ass.net> <20170419153703.GQ3956@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170419153703.GQ3956@linux.vnet.ibm.com> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 19, 2017 at 08:37:03AM -0700, Paul E. McKenney wrote: > On Wed, Apr 19, 2017 at 03:15:53PM +0200, Peter Zijlstra wrote: > > On Wed, Apr 19, 2017 at 06:02:45AM -0700, Paul E. McKenney wrote: > > > On Wed, Apr 19, 2017 at 01:28:45PM +0200, 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. > > > > > > > > I've not looked hard enough at the new SRCU to see if its possible to > > > > re-instate that feature. > > > > > > And with the fix I gave Maz, the parallelized version is near enough > > > to being free as well. It was just a stupid bug on my part: I forgot > > > to check for expedited when scheduling callbacks. > > > > Right, although for the old SRCU it was true for !expedited as well. > > Which is all good fun until someone does a call_srcu() on each and > every munmap() syscall. ;-) Well, that being a different SRCU domain doesn't affect the KVM memslot domain thingy ;-) > But the current code is much better housebroken. ;-) It is. But a workload that manages to hit sync_expedited in a loop on all CPUs is still O(n^2) work. And the more sync_expedited instances we have, the more likely that becomes.