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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 EB655C04AB6 for ; Tue, 28 May 2019 18:23:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D0091208CB for ; Tue, 28 May 2019 18:23:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728009AbfE1SXM (ORCPT ); Tue, 28 May 2019 14:23:12 -0400 Received: from mga09.intel.com ([134.134.136.24]:18639 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727928AbfE1SXM (ORCPT ); Tue, 28 May 2019 14:23:12 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga006.fm.intel.com ([10.253.24.20]) by orsmga102.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 28 May 2019 11:23:11 -0700 X-ExtLoop1: 1 Received: from linux.intel.com ([10.54.29.200]) by fmsmga006.fm.intel.com with ESMTP; 28 May 2019 11:23:11 -0700 Received: from [10.254.95.162] (kliang2-mobl.ccr.corp.intel.com [10.254.95.162]) (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 79E9C5802C9; Tue, 28 May 2019 11:23:10 -0700 (PDT) From: "Liang, Kan" Subject: Re: [PATCH 4/9] 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: <20190521214055.31060-1-kan.liang@linux.intel.com> <20190521214055.31060-5-kan.liang@linux.intel.com> <20190528124349.GU2606@hirez.programming.kicks-ass.net> Message-ID: <287c2c84-25cf-fdce-a3c3-49a6ee93ae4c@linux.intel.com> Date: Tue, 28 May 2019 14:23:09 -0400 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <20190528124349.GU2606@hirez.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 5/28/2019 8:43 AM, Peter Zijlstra wrote: > On Tue, May 21, 2019 at 02:40:50PM -0700, kan.liang@linux.intel.com wrote: >> The 8bit metrics ratio values lose precision when the measurement period >> gets longer. >> >> To avoid this we always reset the metric value when reading, as we >> already accumulate the count in the perf count value. >> >> For a long period read, low precision is acceptable. >> For a short period read, the register will be reset often enough that it >> is not a problem. > >> The PERF_METRICS may report wrong value if its delta was less than 1/255 >> of SLOTS (Fixed counter 3). >> >> To avoid this, the PERF_METRICS and SLOTS registers have to be reset >> simultaneously. The slots value has to be cached as well. > > That doesn't sound like it is NMI-safe. > > > >> RDPMC >> ========= >> The TopDown can be collected per thread/process. To use TopDown >> through RDPMC in applications on Icelake, the metrics and slots values >> have to be saved/restored during context switching. >> >> Add specific set_period() to specially handle the slots and metrics >> event. Because, >> - The initial value must be 0. >> - Only need to restore the value in context switch. For other cases, >> the counters have been cleared after read. > > So the above claims to explain RDPMC, but doesn't mention that magic > value below at all. In fact, I don't see how the above relates to RDPMC > at all. Current perf only support per-core Topdown RDPMC. On Icelake, it can be extended to per-thread Topdown RDPMC. It tries to explain the extra work for per-thread topdown RDPMC, e.g. save/restore slots and metrics value in context switch. > >> @@ -2141,7 +2157,9 @@ static int x86_pmu_event_idx(struct perf_event *event) >> if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED)) >> return 0; >> >> - if (x86_pmu.num_counters_fixed && idx >= INTEL_PMC_IDX_FIXED) { >> + if (is_metric_idx(idx)) >> + idx = 1 << 29; > > I can't find this in the SDM RDPMC description. What does it return? It will return the value of PERF_METRICS. I will add it in the changelog. Thanks, Kan > >> + else if (x86_pmu.num_counters_fixed && idx >= INTEL_PMC_IDX_FIXED) { >> idx -= INTEL_PMC_IDX_FIXED; >> idx |= 1 << 30; >> }