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=-2.3 required=3.0 tests=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 2D39EC3A59F for ; Thu, 29 Aug 2019 13:31:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id F377C233FF for ; Thu, 29 Aug 2019 13:31:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727176AbfH2Nbl (ORCPT ); Thu, 29 Aug 2019 09:31:41 -0400 Received: from mga02.intel.com ([134.134.136.20]:41930 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726518AbfH2Nbk (ORCPT ); Thu, 29 Aug 2019 09:31:40 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga002.jf.intel.com ([10.7.209.21]) by orsmga101.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 29 Aug 2019 06:31:39 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.64,443,1559545200"; d="scan'208";a="192957950" Received: from linux.intel.com ([10.54.29.200]) by orsmga002.jf.intel.com with ESMTP; 29 Aug 2019 06:31:39 -0700 Received: from [10.251.1.23] (kliang2-mobl.ccr.corp.intel.com [10.251.1.23]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by linux.intel.com (Postfix) with ESMTPS id 93B4458046E; Thu, 29 Aug 2019 06:31:38 -0700 (PDT) Subject: Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown metrics To: Peter Zijlstra Cc: acme@kernel.org, mingo@redhat.com, linux-kernel@vger.kernel.org, tglx@linutronix.de, jolsa@kernel.org, eranian@google.com, alexander.shishkin@linux.intel.com, ak@linux.intel.com References: <20190826144740.10163-1-kan.liang@linux.intel.com> <20190826144740.10163-4-kan.liang@linux.intel.com> <20190828151921.GD17205@worktop.programming.kicks-ass.net> From: "Liang, Kan" Message-ID: Date: Thu, 29 Aug 2019 09:31:37 -0400 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20190828151921.GD17205@worktop.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 8/28/2019 11:19 AM, Peter Zijlstra wrote: >> +static int icl_set_topdown_event_period(struct perf_event *event) >> +{ >> + struct hw_perf_event *hwc = &event->hw; >> + s64 left = local64_read(&hwc->period_left); >> + >> + /* >> + * Clear PERF_METRICS and Fixed counter 3 in initialization. >> + * After that, both MSRs will be cleared for each read. >> + * Don't need to clear them again. >> + */ >> + if (left == x86_pmu.max_period) { >> + wrmsrl(MSR_CORE_PERF_FIXED_CTR3, 0); >> + wrmsrl(MSR_PERF_METRICS, 0); >> + local64_set(&hwc->period_left, 0); >> + } > This really doesn't make sense to me; if you set FIXED_CTR3 := 0, you'll > never trigger the overflow there; this then seems to suggest the actual > counter value is irrelevant. Therefore you don't actually need this. > Could you please elaborate on why initialization to 0 never triggers an overflow? As of my understanding, initialization to 0 only means that it will take more time than initialization to -max_period (0x8000 0000 0001) to trigger an overflow. Maybe 0 is too tricky. We can set the initial value to 1. I think the bottom line is that we need a small initial value for FIXED_CTR3 here. PERF_METRICS reports an 8bit integer fraction which is something like 0xff * internal counters / FIXCTR3. The internal counters only start counting from 0. (SW cannot set an arbitrary initial value for internal counters.) If the initial value of FIXED_CTR3 is too big, PERF_METRICS could always remain constant, e.g. 0. Thanks, Kan