Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1422908AbbEEVKS (ORCPT ); Tue, 5 May 2015 17:10:18 -0400 Received: from mx1.redhat.com ([209.132.183.28]:44466 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1161610AbbEEVKN (ORCPT ); Tue, 5 May 2015 17:10:13 -0400 Message-ID: <55493183.2070201@redhat.com> Date: Tue, 05 May 2015 17:09:23 -0400 From: Rik van Riel User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0 MIME-Version: 1.0 To: paulmck@linux.vnet.ibm.com, Peter Zijlstra CC: 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 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> <20150505183556.GS5381@linux.vnet.ibm.com> In-Reply-To: <20150505183556.GS5381@linux.vnet.ibm.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2000 Lines: 44 On 05/05/2015 02:35 PM, Paul E. McKenney wrote: > 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. Interrupts are already disabled on kernel-user and kernel-guest switches. Paolo and I have patches to move a bunch of the calls to user_enter, user_exit, guest_enter, and guest_exit to places where interrupts are already disabled, so we do not need to disable them again. With those in place, the vtime calculations are the largest CPU user. I am working on those. -- All rights reversed -- 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/