Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752941AbbHCOL6 (ORCPT ); Mon, 3 Aug 2015 10:11:58 -0400 Received: from casper.infradead.org ([85.118.1.10]:48812 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751974AbbHCOL5 (ORCPT ); Mon, 3 Aug 2015 10:11:57 -0400 Date: Mon, 3 Aug 2015 16:11:52 +0200 From: Peter Zijlstra To: Frederic Weisbecker Cc: LKML , Thomas Gleixner , Preeti U Murthy , Christoph Lameter , Ingo Molnar , Viresh Kumar , Rik van Riel Subject: Re: [PATCH 05/10] nohz: New tick dependency mask Message-ID: <20150803141152.GS25159@twins.programming.kicks-ass.net> References: <1437669735-8786-1-git-send-email-fweisbec@gmail.com> <1437669735-8786-6-git-send-email-fweisbec@gmail.com> <20150803125717.GJ19282@twins.programming.kicks-ass.net> <20150803130938.GD20269@lerouge> <20150803132958.GQ25159@twins.programming.kicks-ass.net> <20150803135532.GB25554@lerouge> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150803135532.GB25554@lerouge> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1996 Lines: 58 On Mon, Aug 03, 2015 at 03:55:34PM +0200, Frederic Weisbecker wrote: > On Mon, Aug 03, 2015 at 03:29:58PM +0200, Peter Zijlstra wrote: > > On Mon, Aug 03, 2015 at 03:09:39PM +0200, Frederic Weisbecker wrote: > > > > > > That doesn't make any sense: > > > > > > > > tick_nohz_set_tick_dependency_this_cpu(); > > > > > > > > (shees, you're nowhere near lazy enough, that's insane to type) is > > > > almost identical to: > > > > > > > > tick_nohz_set_tick_dependency_cpu(.cpu = smp_processor_id()); > > > > > > > > The only difference is a _very_ slight reduction in cost for computing > > > > the per-cpu offset. > > > > > > But the local one must be NMI-safe. Now I can do: > > > > > > if (cpu == smp_processor_id()) > > > tick_nohz_full_kick() // NMI-safe > > > else > > > tick_nohz_full_kick_cpu(cpu); // not NMI-safe. > > > > Urgh, I missed that. But yes, I suppose that's ok seeing how we result > > in a smaller interface. > > > > I was going to say that with a bit of luck GCC could optimize it, but > > its not inline so no it cannot. > > I might inline all these set_dep() things to introduce static keys on these > APIs.. But the kick itself will remain real calls. Sure, but first check if GCC will optimize: static inline void foo(int cpu) { if (cpu == smp_processor_id()) bar1(); else bar2(); } foo(smp_processor_id()); Into a direct call to bar1(), if not see if we can make it so. If not, there's no point in inlining at all. > Ok how about tick_nohz_set_dep_nmi() so that we know exactly what's the purpose > here. Still a long function name but it's clear. Only for the set, if you really care about it. The alternative is WARN_ON(in_nmi() && cpu != smp_processor_id()) or somesuch. -- 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/