From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6031BC3A59F for ; Sat, 31 Aug 2019 09:29:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 31A2F2087E for ; Sat, 31 Aug 2019 09:29:34 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="Z5GopLAK" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726498AbfHaJ32 (ORCPT ); Sat, 31 Aug 2019 05:29:28 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:41068 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726184AbfHaJ32 (ORCPT ); Sat, 31 Aug 2019 05:29:28 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=KmR/c4ArTiru3qyk+20TYV+nkGr/yqWbxKniMkdJ5RQ=; b=Z5GopLAK//pWlmGZC0W1UOjiM Q8o7ry9QSbPDGazdsuNAy0nl7ihSFMqZQcjds4bMS2mdw/meSo+bzAgQxHW7I/sK5VPZIJbbRnps6 TxvovqVkyTmvE9KEiC/cs0FCXMF4v+9LCYIFZSXuTe9uvS8aCHEzwtBPK8bikCGPN4iZrosrvemKH U5TifBQzUyt0LrQc8WdQczV+emqbFC/+3aqPiEz6pA9l7Yg5u60dQTzqtT4rS3jjRMlqQgB0z7vWC lACy0RJTvfehGz/91ri+DNlJGg0zwNXP0nPhzLcMska/y18yST1iySUHebhcJvVXBxwfUn7oLTy/s AKy0+s8lQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.92 #3 (Red Hat Linux)) id 1i3zhD-0004WD-Mj; Sat, 31 Aug 2019 09:29:23 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 0E794300B8D; Sat, 31 Aug 2019 11:28:47 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id ED7C729B2CD2F; Sat, 31 Aug 2019 11:29:21 +0200 (CEST) Date: Sat, 31 Aug 2019 11:29:21 +0200 From: Peter Zijlstra To: Stephane Eranian Cc: Andi Kleen , "Liang, Kan" , Arnaldo Carvalho de Melo , Ingo Molnar , LKML , Thomas Gleixner , Jiri Olsa , Alexander Shishkin Subject: Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown metrics Message-ID: <20190831092921.GK2369@hirez.programming.kicks-ass.net> References: <20190826144740.10163-1-kan.liang@linux.intel.com> <20190826144740.10163-4-kan.liang@linux.intel.com> <20190831003110.GA5447@tassilo.jf.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Aug 31, 2019 at 02:13:05AM -0700, Stephane Eranian wrote: > With PERF_METRICS, the delta is always since previous read. If you read > frequently enough you do not lose precision. You always loose precision, the whole fraction of 255 looses the fractional part; consider: 255 * 1 / 8 31.87500000000000000000 255 * 7 / 8 223.12500000000000000000 31 * 8 / 255 .97254901960784313725 223 * 8 / 255 6.99607843137254901960 Now, we can make reconstruction use normal 'round-nearest' and then we'd get 1 and 7 back, but there's always cases where you loose things. Also, with 255 being an odd number, round-nearest is actually 'hard' :/