Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756943AbbEESxf (ORCPT ); Tue, 5 May 2015 14:53:35 -0400 Received: from e31.co.us.ibm.com ([32.97.110.149]:49840 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752105AbbEESxK (ORCPT ); Tue, 5 May 2015 14:53:10 -0400 Date: Tue, 5 May 2015 11:35:56 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Rik van Riel , Paolo Bonzini , Ingo Molnar , Andy Lutomirski , "linux-kernel@vger.kernel.org" , X86 ML , williams@redhat.com, Andrew Lutomirski , fweisbec@redhat.com, Heiko Carstens , Thomas Gleixner , Ingo Molnar , Linus Torvalds Subject: Re: question about RCU dynticks_nesting Message-ID: <20150505183556.GS5381@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <5543CFE5.1030509@redhat.com> <20150502052733.GA9983@gmail.com> <55473B47.6080600@redhat.com> <55479749.7070608@redhat.com> <5547C1DC.10802@redhat.com> <20150504193923.GX5381@linux.vnet.ibm.com> <20150505105346.GJ21418@twins.programming.kicks-ass.net> <20150505123446.GN5381@linux.vnet.ibm.com> <20150505130026.GM21418@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150505130026.GM21418@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15050518-8236-0000-0000-00000B3EE58F Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1578 Lines: 34 On Tue, May 05, 2015 at 03:00:26PM +0200, Peter Zijlstra wrote: > On Tue, May 05, 2015 at 05:34:46AM -0700, Paul E. McKenney wrote: > > On Tue, May 05, 2015 at 12:53:46PM +0200, Peter Zijlstra wrote: > > > On Mon, May 04, 2015 at 12:39:23PM -0700, Paul E. McKenney wrote: > > > > But in non-preemptible RCU, we have PREEMPT=n, so there is no preempt > > > > counter in production kernels. Even if there was, we have to sample this > > > > on other CPUs, so the overhead of preempt_disable() and preempt_enable() > > > > would be where kernel entry/exit is, so I expect that this would be a > > > > net loss in overall performance. > > > > > > We unconditionally have the preempt_count, its just not used much for > > > PREEMPT_COUNT=n kernels. > > > > We have the field, you mean? I might be missing something, but it still > > appears to me thta preempt_disable() does nothing for PREEMPT=n kernels. > > So what am I missing? > > There's another layer of accessors that can in fact manipulate the > preempt_count even for !PREEMPT_COUNT kernels. They are currently used > by things like pagefault_disable(). OK, fair enough. I am going to focus first on getting rid of (or at least greatly reducing) RCU's interrupt disabling on the user-kernel entry/exit paths, since that seems to be the biggest cost. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/