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 18C9AC28CC0 for ; Thu, 30 May 2019 16:25:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DF6B62492B for ; Thu, 30 May 2019 16:25:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727359AbfE3QZB (ORCPT ); Thu, 30 May 2019 12:25:01 -0400 Received: from mga11.intel.com ([192.55.52.93]:24365 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726355AbfE3QZB (ORCPT ); Thu, 30 May 2019 12:25:01 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga008.fm.intel.com ([10.253.24.58]) by fmsmga102.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 30 May 2019 09:25:01 -0700 X-ExtLoop1: 1 Received: from linux.intel.com ([10.54.29.200]) by fmsmga008.fm.intel.com with ESMTP; 30 May 2019 09:25:00 -0700 Received: from [10.251.91.204] (abudanko-mobl.ccr.corp.intel.com [10.251.91.204]) by linux.intel.com (Postfix) with ESMTP id 5288758010A; Thu, 30 May 2019 09:24:58 -0700 (PDT) Subject: Re: [PATCH v4] perf record: collect user registers set jointly with dwarf stacks To: Arnaldo Carvalho de Melo Cc: Jiri Olsa , Namhyung Kim , Alexander Shishkin , Peter Zijlstra , Ingo Molnar , Andi Kleen , linux-kernel References: <01a322ee-c99d-0bb7-b7cf-bc1fa8064d75@linux.intel.com> <20190529192506.GB5553@kernel.org> <378b81a7-b7db-c60f-134d-0c0f7cd6c0a1@linux.intel.com> <20190530131337.GB21962@kernel.org> From: Alexey Budankov Organization: Intel Corp. Message-ID: Date: Thu, 30 May 2019 19:24:57 +0300 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: <20190530131337.GB21962@kernel.org> Content-Type: text/plain; charset=utf-8 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 30.05.2019 16:13, Arnaldo Carvalho de Melo wrote: > Em Thu, May 30, 2019 at 11:24:49AM +0300, Alexey Budankov escreveu: >> On 29.05.2019 22:25, Arnaldo Carvalho de Melo wrote: >>> Em Wed, May 29, 2019 at 05:30:49PM +0300, Alexey Budankov escreveu: >> >>>> +++ b/tools/perf/util/evsel.c >>>> +#define DWARF_REGS_MASK ((1ULL << PERF_REG_IP) | \ >>>> + (1ULL << PERF_REG_SP)) >>>> + >>>> static void __perf_evsel__config_callchain(struct perf_evsel *evsel, >>>> struct record_opts *opts, >>>> struct callchain_param *param) >>>> @@ -702,7 +705,13 @@ static void __perf_evsel__config_callchain(struct perf_evsel *evsel, >>>> if (!function) { >>>> perf_evsel__set_sample_bit(evsel, REGS_USER); >>>> perf_evsel__set_sample_bit(evsel, STACK_USER); >>>> - attr->sample_regs_user |= PERF_REGS_MASK; >>>> + if (opts->sample_user_regs) { >>> >>> Where are you checking that opts->sample_user_regs doesn't have either >>> IP or SP? >> >> Sure. The the intention was to avoid such a complication, merge two >> masks and provide explicit warning that the resulting mask is extended. > > s/is/may be/g > >> If you still see the checking and auto detection of the exact mask >> extension as essential it can be implemented. > > perf, tracing, systems internals, etc are super complicated, full of > details, the more precise we can make the messages, the better. > >>> So, __perf_evsel__config_callchain its the routine that sets up the >>> attr->sample_regs_user when callchains are asked for, and what was it >>> doing? Asking for _all_ user regs, right? >>> >>> I.e. what you're saying is that when --callgraph-dwarf is asked for, >>> then only IP and BP are needed, and we should stop doing that, so that >>> would be a first patch, if that is the case. I.e. a patch that doesn't >>> even mention opts->sample_user_regs. >>> >>> Then, a second patch would fix the opt->sample_user_regs request clash >>> with --callgraph dwarf, i.e. it would do something like: >>> >>> if ((opts->sample_regs_user & DWARF_REGS_MASK) != DWARF_REGS_MASK) { >>> char * ip = (opts->sample_regs_user & (1ULL << PERF_REG_IP)) ? NULL : "IP", >>> * sp = (opts->sample_regs_user & (1ULL << PERF_REG_SP)) ? NULL : "SP", >>> * all = (!ip && !sp) ? "s" : ""; >>> >>> pr_warning("WARNING: specified --user-regs register set doesn't include register%s " >>> "needed by also specified --call-graph=dwarf, auto adding %s%s%s register%s.\n", >>> all, ip, all : ", " : "", sp, all); >>> } >>> >>> This if and only if all the registers that are needed to do DWARF >>> unwinding are just IP and BP, which doesn't look like its true, since >>> when no --user_regs is set (i.e. opts->user_regs is not set) then we >>> continue asking for PERF_REGS_MASK... >>> >>> Can you check where I'm missing something? >> >> 1. -g call-graph dwarf,K full_regs >> 2. --user-regs=user_regs user_regs >> 3. -g call-graph dwarf,K --user-regs=user_regs user_regs + dwarf_regs >> >> The default behavior stays the same for cases 1, 2 above. >> For case 3 register set becomes the one asked using --user_regs option. >> If the option value misses IP or SP or the both then they are explicitly >> added to the option value and a warning message mentioning the exact >> added registers is provided. > >>> Jiri DWARF unwind uses just IP and SP? Looking at >>> tools/perf/util/unwind-libunwind-local.c's access_reg() I don't think >>> so, right? > >> If you ask me, AFAIK, DWARF unwind rules sometimes can refer additional >> general purpose registers for frames boundaries calculation. > > :-) So that DWARF_REGS is misleading, should be something like > DWARF_MINIMAL_REGS, as we may need other registers, so the original code > was correct, right? Right. Actually came to the same conclusion with the same naming for IP,SP mask :) > > After all if the user asks for both --call-graph dwarf and --user-regs, > then probably we should require --force? I.e. the message then would be: > > " > WARNING: The use of --call-graph=dwarf may require all the user > registers, specifying a subset with --user-regs may render DWARF > unwinding unreliable, please use --force if you're sure that the subset > specified via --user-regs is enough for your specific use case. > " > > And then plain refuse, if the user _really_ wants it, then we have > --force/-f for those cases. > > Does this sound better? If --user-regs is specified jointly with --call-graph dwarf option then --user-regs already serves as the --force and, IMHO, a warning does the best. The ideal solution, I could imagine, is to also dynamically calculate regs set extension and provide it in the warning, but it is only for two registers. So, if --call-graph dwarf --user-regs=A,B,C are specified jointly then " WARNING: The use of --call-graph=dwarf may require all the user registers, specifying a subset with --user-regs may render DWARF unwinding unreliable, so the minimal registers set (IP, SP) is explicitly forced. " The message is precise and it would fit the majority of use cases. Final decision is up to you. ~Alexey > > - Arnaldo >