Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752622AbaJFM2e (ORCPT ); Mon, 6 Oct 2014 08:28:34 -0400 Received: from www.linutronix.de ([62.245.132.108]:54057 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752167AbaJFM2c (ORCPT ); Mon, 6 Oct 2014 08:28:32 -0400 Date: Mon, 6 Oct 2014 14:28:26 +0200 (CEST) From: Thomas Gleixner To: David Lang cc: Christoph Lameter , Richard Cochran , linux-kernel@vger.kernel.org Subject: Re: Why do we still have 32 bit counters? Interrupt counters overflow within 50 days In-Reply-To: Message-ID: References: <20141003120345.GA6652@localhost.localdomain> User-Agent: Alpine 2.11 (DEB 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 6 Oct 2014, David Lang wrote: > On Sun, 5 Oct 2014, Thomas Gleixner wrote: > > > > > > > 64 bit counters on 32 bit machines are not an easy thing and could be > > > > Whats so hard about 64bit counters on 32bit machines? > > > > > expensive to handle in particular because these counters are used in > > > performance critical hotpaths. > > > > The expensive overhead is a single "adcl" instruction. > > As I understand it, since the 64 bit math cannot be made atomic, it requires > protecting the counter with a lock so that it can't be read while half > updated. Aquiring a lock on every update is an expensive thing to do. It's not > something people like to see in a fast path, especially for something of as > low an importance as the counters. As I said before. Both reader and writer side are already protected by the irq descriptor lock. We take that lock anyway. Thanks, tglx -- 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/