Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753172Ab1DWMG5 (ORCPT ); Sat, 23 Apr 2011 08:06:57 -0400 Received: from smtp-out.google.com ([74.125.121.67]:58072 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751221Ab1DWMGy convert rfc822-to-8bit (ORCPT ); Sat, 23 Apr 2011 08:06:54 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=google.com; s=beta; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type:content-transfer-encoding; b=wx4RZCP7Dp4IgDXibMyoXUfsWBs4aWyVO6etP+0d4x2ezZGFMn0a2XIlINk9USxXws 9OxtamcXKov56uoI0dow== MIME-Version: 1.0 In-Reply-To: <1303545012.2298.44.camel@twins> References: <20110422092322.GA1948@elte.hu> <20110422105211.GB1948@elte.hu> <20110422165007.GA18401@vps.sharma-home.net> <20110422203022.GA20573@elte.hu> <20110422203222.GA21219@elte.hu> <20110423000347.GC9328@tassilo.jf.intel.com> <1303545012.2298.44.camel@twins> Date: Sat, 23 Apr 2011 14:06:47 +0200 Message-ID: Subject: Re: [generalized cache events] Re: [PATCH 1/1] perf tools: Add missing user space support for config1/config2 From: Stephane Eranian To: Peter Zijlstra Cc: Andi Kleen , Ingo Molnar , arun@sharma-home.net, Arnaldo Carvalho de Melo , linux-kernel@vger.kernel.org, Lin Ming , Arnaldo Carvalho de Melo , Thomas Gleixner , eranian@gmail.com, Arun Sharma , Linus Torvalds , Andrew Morton Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1790 Lines: 36 On Sat, Apr 23, 2011 at 9:50 AM, Peter Zijlstra wrote: > On Fri, 2011-04-22 at 17:03 -0700, Andi Kleen wrote: >> > > Yes, and note that with instructions events we even have skid-less PEBS >> > > profiling so seeing the precise . >> >                                   - location of instructions is possible. >> >> It was better when it was eaten. PEBS does not actually eliminated >> skid unfortunately. The interrupt still occurs later, so the >> instruction location is off. >> >> PEBS merely gives you more information. > > You're so skilled at not actually saying anything useful. Are you > perchance referring to the fact that the IP reported in the PEBS data is > exactly _one_ instruction off? Something that is demonstrated to be > fixable? > > Or are you defining skid differently and not telling us your definition? > PEBS is guaranteed to return an IP that is just after AN instruction that caused the event. However, that instruction is NOT the one at the end of your period. Let's take an example with INST_RETIRED, period=100000. Then, the IP you get is NOT after the 100,000th retired instruction. It's an instruction that is N cycles after that one. There is internal skid due to the way PEBS is implemented. That is what Andi is referring to. The issue causes bias and thus impacts the quality of the samples. On SNB, there is a new INST_RETIRED:PREC_DIST event. PREC_DIST=precise distribution. It tries to correct for the skid on this event on INST_RETIRED with PEBS (look at Vol3b). -- 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/