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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 95F02C33CB3 for ; Wed, 15 Jan 2020 12:57:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 75FDF2073A for ; Wed, 15 Jan 2020 12:57:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728884AbgAOM5P convert rfc822-to-8bit (ORCPT ); Wed, 15 Jan 2020 07:57:15 -0500 Received: from eu-smtp-delivery-151.mimecast.com ([146.101.78.151]:26466 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726071AbgAOM5P (ORCPT ); Wed, 15 Jan 2020 07:57:15 -0500 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-229-8hDPppHqO_qsVCTS73igrw-1; Wed, 15 Jan 2020 12:57:11 +0000 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Wed, 15 Jan 2020 12:57:10 +0000 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Wed, 15 Jan 2020 12:57:10 +0000 From: David Laight To: 'Steven Rostedt' CC: 'Vincent Guittot' , Peter Zijlstra , Viresh Kumar , Ingo Molnar , Juri Lelli , Dietmar Eggemann , Ben Segall , Mel Gorman , linux-kernel Subject: RE: sched/fair: scheduler not running high priority process on idle cpu Thread-Topic: sched/fair: scheduler not running high priority process on idle cpu Thread-Index: AdXK8cUFXa7JpPXmQNq7oQ32S9fYHAACik4AAADJLkAAAO3PAAAnruzA Date: Wed, 15 Jan 2020 12:57:10 +0000 Message-ID: <3960d46b3a4a4053a696a98ee6fd131d@AcuMS.aculab.com> References: <212fabd759b0486aa8df588477acf6d0@AcuMS.aculab.com> <20200114115906.22f952ff@gandalf.local.home> <5ba2ae2d426c4058b314c20c25a9b1d0@AcuMS.aculab.com> <20200114124812.4d5355ae@gandalf.local.home> In-Reply-To: <20200114124812.4d5355ae@gandalf.local.home> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 X-MC-Unique: 8hDPppHqO_qsVCTS73igrw-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Steven Rostedt > Sent: 14 January 2020 17:48 ... > > The cost of ftrace function call entry/exit (about 200 clocks) makes it > > rather unsuitable for any performance measurements unless only > > a very few functions are traced - which rather requires you know > > what the code is doing :-( > > > > Well, when I use function tracing, I start all of them, analyze the > trace, then the functions I don't care about (usually spin locks and > other utils), I add to the set_ftrace_notrace file, which keeps them > from being part of the trace. I keep doing this until I find a set of > functions that doesn't hurt overhead as much and gives me enough > information to know what is happening. It also helps to enable all or > most events (at least scheduling events). I've been using schedviz - but have had to 'fixup' wrapped traces so that all the cpu traces start at the same time to get it to load them. I managed to find what the worker thread was running - but only because it ran for the entire time 'echo t >/proc/sysrq-trigger' took to finish. Then I looked at the sources to find the code... I'm surprised the 'normal case' for tracing function entry isn't done in assembler without saving all the registers (etc). For tsc stamps I think it should be possible saving just 3 registers in under 32 instructions. Scaling to ns is a bit harder. It's a shame the ns scaling isn't left to the reading code. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)