Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754140Ab1DSSVu (ORCPT ); Tue, 19 Apr 2011 14:21:50 -0400 Received: from one.firstfloor.org ([213.235.205.2]:56508 "EHLO one.firstfloor.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752384Ab1DSSVt (ORCPT ); Tue, 19 Apr 2011 14:21:49 -0400 Date: Tue, 19 Apr 2011 20:21:48 +0200 From: Andi Kleen To: Robert Richter Cc: Andi Kleen , Peter Zijlstra , Ingo Molnar , Stephane Eranian , LKML Subject: Re: [PATCH 3/4] perf, x86: Use ALTERNATIVE() to check for X86_FEATURE_PERFCTR_CORE Message-ID: <20110419182148.GS16484@one.firstfloor.org> References: <1302913676-14352-1-git-send-email-robert.richter@amd.com> <1302913676-14352-4-git-send-email-robert.richter@amd.com> <20110419103927.GV31407@erda.amd.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110419103927.GV31407@erda.amd.com> User-Agent: Mutt/1.4.2.2i Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 960 Lines: 24 On Tue, Apr 19, 2011 at 12:39:27PM +0200, Robert Richter wrote: > On 18.04.11 16:00:57, Andi Kleen wrote: > > Robert Richter writes: > > > > > Using ALTERNATIVE() when checking for X86_FEATURE_PERFCTR_CORE avoids > > > an extra pointer chase and data cache hit. > > > > Is that really a performance critical path? > > > > Seems more like unnecessary obfuscation to me. > > We hotest path is in perf_pmu_disable(), which happens at least with > every task switch when calling the event scheduler. Yes but that's already a slow path isn't it? It better is, because the MSR accesses alone are incredibly expensive. I guess your test and jump isn't even on the radar after that ... -Andi -- 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/