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 0D256C433FE for ; Wed, 23 Mar 2022 20:54:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1344836AbiCWU4I (ORCPT ); Wed, 23 Mar 2022 16:56:08 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33086 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S244002AbiCWU4I (ORCPT ); Wed, 23 Mar 2022 16:56:08 -0400 Received: from mail-lf1-f45.google.com (mail-lf1-f45.google.com [209.85.167.45]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E21338BF15; Wed, 23 Mar 2022 13:54:36 -0700 (PDT) Received: by mail-lf1-f45.google.com with SMTP id bt26so4744068lfb.3; Wed, 23 Mar 2022 13:54:36 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=PsL8QsEp/CuRBHIlYfHFpv5xpQDqvATabhPlJFg6D18=; b=w8A+ABiQz3SR3oWquM8y/Z4/b/F0HrS/YHNgSXf9TDUmIoTdK/gbGLYY+FeZQ44qmS V9SeTeNnunzXekutPDl5oMUHA8YFNDQ18jUf6O7DFZDvD3bmZsUXDgwlNoTcFGUoa6Xo h3pZfPljm1UaEFa4bomIs58GE0Ko6gokDYD94u5Hz0MGia0xZLs4DTDbb0JJYdU2jj6+ otwDV0ODsCSTltvtx+h5BUVX401GsxHQDSbreZolhurMnV1acRBRSA8fzASDR/WwqVz1 InBGPOdXlIuVoryzZnjYYxmxZ75+YDb9C7b07QqWq/wMj934uT/YPuFusxRWOXkSsdTP JXWg== X-Gm-Message-State: AOAM5309KmARzUymRTanpS5LiRvPffVaXGGlSJ55ofOvVvgKS2TNb0Gu Hu09TySEnRQyqtuf8LNjx+SSzakqsreaAdkFexaBHiL6 X-Google-Smtp-Source: ABdhPJyx4RD0OigisgS3C9D66vjkzcPeC7rPv4U8Ard06XHOzvE+BtP6nvSHOEIPvkY9ldL4xFdwrixerGDN1jduk6Y= X-Received: by 2002:a05:6512:3404:b0:44a:310f:72f7 with SMTP id i4-20020a056512340400b0044a310f72f7mr1321451lfr.47.1648068875123; Wed, 23 Mar 2022 13:54:35 -0700 (PDT) MIME-Version: 1.0 References: <20220317155346.577384-1-tmricht@linux.ibm.com> In-Reply-To: From: Namhyung Kim Date: Wed, 23 Mar 2022 13:54:23 -0700 Message-ID: Subject: Re: [PATCH] perf/stat: Fix perf stat for forked applications To: Jiri Olsa Cc: Thomas Richter , linux-kernel , linux-perf-users , Arnaldo Carvalho de Melo , gor@linux.ibm.com, Arnaldo Carvalho de Melo , Sven Schnelle , sumanthk@linux.ibm.com, Heiko Carstens Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-perf-users@vger.kernel.org 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, Namhyung