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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id D87B4C433FE for ; Thu, 24 Mar 2022 20:36:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1353445AbiCXUi1 (ORCPT ); Thu, 24 Mar 2022 16:38:27 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49194 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1346156AbiCXUiZ (ORCPT ); Thu, 24 Mar 2022 16:38:25 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 49622B0D37; Thu, 24 Mar 2022 13:36:53 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id D8DF760F88; Thu, 24 Mar 2022 20:36:52 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1B981C340EC; Thu, 24 Mar 2022 20:36:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1648154212; bh=Kx6tBROKdcOdeiKP7X4AbU2NzC0vd6LCDCQ6b1g3bQk=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=SvPapQeGsj/0s8K1VaONuL2jEDQcMsPYMoapQ4/y5R3vHXZqkq3cpWbj4lY2BaTX1 eEIjoAH7+ICIjcDFwNx1vpv/yu5O/ju51k7VNa1kZhw5C/TfvFGdox6eC3CtCD7zBV 1lP9nAQU4U6OjjmvQmZX0U5aNXQRsKRhKmUwgw3nLrn+jCpmy+BrcpgLgY6CSBXhud ViNkzdJsjcaEJJVcFVRxNlgVGTXlUAiEkw+PN4SQEJODMIFjHmyxUO3oiyIthH4VuV lTfI4FGlB438hp4zx33DAfcgOsKkIV74Z5TkulgmVtPP8b3DyuO4D08qCMnaN96j3y YchcITBpma4pQ== Received: by quaco.ghostprotocols.net (Postfix, from userid 1000) id 10E8C40407; Thu, 24 Mar 2022 17:36:50 -0300 (-03) Date: Thu, 24 Mar 2022 17:36:50 -0300 From: Arnaldo Carvalho de Melo To: Namhyung Kim Cc: Jiri Olsa , Thomas Richter , linux-kernel , linux-perf-users , gor@linux.ibm.com, Arnaldo Carvalho de Melo , Sven Schnelle , sumanthk@linux.ibm.com, Heiko Carstens Subject: Re: [PATCH] perf/stat: Fix perf stat for forked applications Message-ID: References: <20220317155346.577384-1-tmricht@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Url: http://acmel.wordpress.com Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Em Wed, Mar 23, 2022 at 01:54:23PM -0700, Namhyung Kim escreveu: > On Wed, Mar 23, 2022 at 4:07 AM Jiri Olsa wrote: > > > > On Thu, Mar 17, 2022 at 04:53:46PM +0100, Thomas Richter wrote: > > > I have run into the following issue: > > > > > > # perf stat -a -e new_pmu/INSTRUCTION_7/ -- mytest -c1 7 > > > > > > Performance counter stats for 'system wide': > > > > > > 0 new_pmu/INSTRUCTION_7/ > > > > > > 0.000366428 seconds time elapsed > > > # > > > > > > The new PMU for s390 counts the execution of certain CPU instructions. > > > The root cause is the extremely small run time of the > > > mytest program. It just executes some assembly instructions > > > and then exits. In above invocation the instruction is executed > > > exactly one time (-c1 option). The PMU is expected to report this one > > > time execution by a counter value of one, but fails to do so > > > in some cases, not all. > > > > > > Debugging reveals the invocation of the child process is done > > > *before* the counter events are installed and enabled. Tracing > > > reveals that sometimes the child process starts and exits before > > > the event is installed on all CPUs. The more CPUs the machine has, > > > the more often this miscount happens. > > > > > > Fix this by reversing the start of the work load after the events > > > have been installed on the specified CPUs. Now the comment also > > > matches the code. > > > > > > Output after: > > > # perf stat -a -e new_pmu/INSTRUCTION_7/ -- mytest -c1 7 > > > > > > Performance counter stats for 'system wide': > > > > > > 1 new_pmu/INSTRUCTION_7/ > > > > > > 0.000366428 seconds time elapsed > > > # > > > > > > Now the correct result is reported rock solid all the time regardless > > > how many CPUs are online. > > > > > > Fixes: acf2892270dc ("perf stat: Use perf_evlist__prepare/start_workload()) > > > > > > Signed-off-by: Thomas Richter > > > Acked-by: Sumanth Korikkar > > > Cc: Namhyung Kim > > > --- > > > tools/perf/builtin-stat.c | 2 +- > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c > > > index 3f98689dd687..60baa3dadc4b 100644 > > > --- a/tools/perf/builtin-stat.c > > > +++ b/tools/perf/builtin-stat.c > > > @@ -955,10 +955,10 @@ static int __run_perf_stat(int argc, const char **argv, int run_idx) > > > * Enable counters and exec the command: > > > */ > > > if (forks) { > > > - evlist__start_workload(evsel_list); > > > err = enable_counters(); > > > if (err) > > > return -1; > > > + evlist__start_workload(evsel_list); > > > > right, without -a the event has enable_on_exec so the race does not > > matter, but it's a problem for system wide with fork > > Agreed. Also we may move the enable_counters() and the > clock code out of the if block to be shared with the else block. > > Acked-by: Namhyung Kim Thanks, applied and added your comments as "Reviewer notes", and please consider using the Reviewed-by tag instead of the Acked-by ones. Thanks anyways, as always! - Arnaldo