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=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 227BDC76188 for ; Fri, 19 Jul 2019 13:47:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EC24B2173B for ; Fri, 19 Jul 2019 13:47:33 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="axLrHVbl" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728985AbfGSNrc (ORCPT ); Fri, 19 Jul 2019 09:47:32 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:58472 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726243AbfGSNrc (ORCPT ); Fri, 19 Jul 2019 09:47:32 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=+Hokg8oUCnTJMdm04EjJEy1G6bBUYjC7FCjqYdguhZU=; b=axLrHVblj6QSmD96ZBPWWhsJ7 ZTNj+ezWX4oz/SJj96v9eF1lNLXZD3Tcmeq0s0Ibn0Om7SNSHLlJY0j0awyDs2pP7INXSVnVwIzKi qvFs3W15ircRizWGrn1Vz9ddCg9IFrkXJeHv+kK9tDpRIe4DXZBD9UP6Gcwm9Ne3LSRtwcliVLWvQ TMco3z8JAlX27HCKEdYvuFSJBf4Uihx58ZhJ4yhuI8CDOUS4NnkQZfsLSSDOBuPuL3/P+dPq/ptmf exyAQ+twVE8tg/sHgus0qBOev9FkWPIwOPI2VrRkaTc+qAoFgE7GRbEQqdMqfnUNDlAywrV0Dxthi /+fIfHq+Q==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.92 #3 (Red Hat Linux)) id 1hoTEP-0001Lb-5Q; Fri, 19 Jul 2019 13:47:29 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 6B95820B4B7B6; Fri, 19 Jul 2019 15:47:27 +0200 (CEST) Date: Fri, 19 Jul 2019 15:47:27 +0200 From: Peter Zijlstra To: Oleg Nesterov Cc: Ingo Molnar , Thomas Gleixner , Andrew Fox , Stephen Johnston , linux-kernel@vger.kernel.org, Linus Torvalds , Stanislaw Gruszka Subject: Re: [PATCH] sched/cputime: make scale_stime() more precise Message-ID: <20190719134727.GV3463@hirez.programming.kicks-ass.net> References: <20190718131834.GA22211@redhat.com> <20190719110349.GG3419@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190719110349.GG3419@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 19, 2019 at 01:03:49PM +0200, Peter Zijlstra wrote: > On Thu, Jul 18, 2019 at 03:18:34PM +0200, Oleg Nesterov wrote: > > People report that utime and stime from /proc//stat become very wrong > > when the numbers are big enough. In particular, the monitored application > > can run all the time in user-space but only stime grows. > > > > This is because scale_stime() is very inaccurate. It tries to minimize the > > relative error, but the absolute error can be huge. > > > > Andrew wrote the test-case: > > > > int main(int argc, char **argv) > > { > > struct task_cputime c; > > struct prev_cputime p; > > u64 st, pst, cst; > > u64 ut, put, cut; > > u64 x; > > int i = -1; // one step not printed > > > > if (argc != 2) > > { > > printf("usage: %s \n", argv[0]); > > return 1; > > } > > x = strtoull(argv[1], NULL, 0) * SEC; > > printf("start=%lld\n", x); > > > > p.stime = 0; > > p.utime = 0; > > > > while (i++ < NSTEPS) > > { > > x += STEP; > > c.stime = x; > > c.utime = x; > > c.sum_exec_runtime = x + x; > > pst = cputime_to_clock_t(p.stime); > > put = cputime_to_clock_t(p.utime); > > cputime_adjust(&c, &p, &ut, &st); > > cst = cputime_to_clock_t(st); > > cut = cputime_to_clock_t(ut); > > if (i) > > printf("ut(diff)/st(diff): %20lld (%4lld) %20lld (%4lld)\n", > > cut, cut - put, cst, cst - pst); > > } > > } > > > > For example, > > > > $ ./stime 300000 > > start=300000000000000 > > ut(diff)/st(diff): 299994875 ( 0) 300009124 (2000) > > ut(diff)/st(diff): 299994875 ( 0) 300011124 (2000) > > ut(diff)/st(diff): 299994875 ( 0) 300013124 (2000) > > ut(diff)/st(diff): 299994875 ( 0) 300015124 (2000) > > ut(diff)/st(diff): 299994875 ( 0) 300017124 (2000) > > ut(diff)/st(diff): 299994875 ( 0) 300019124 (2000) > > ut(diff)/st(diff): 299994875 ( 0) 300021124 (2000) > > ut(diff)/st(diff): 299994875 ( 0) 300023124 (2000) > > ut(diff)/st(diff): 299994875 ( 0) 300025124 (2000) > > ut(diff)/st(diff): 299994875 ( 0) 300027124 (2000) > > ut(diff)/st(diff): 299994875 ( 0) 300029124 (2000) > > ut(diff)/st(diff): 299996875 (2000) 300029124 ( 0) > > ut(diff)/st(diff): 299998875 (2000) 300029124 ( 0) > > ut(diff)/st(diff): 300000875 (2000) 300029124 ( 0) > > ut(diff)/st(diff): 300002875 (2000) 300029124 ( 0) > > ut(diff)/st(diff): 300004875 (2000) 300029124 ( 0) > > ut(diff)/st(diff): 300006875 (2000) 300029124 ( 0) > > ut(diff)/st(diff): 300008875 (2000) 300029124 ( 0) > > ut(diff)/st(diff): 300010875 (2000) 300029124 ( 0) > > ut(diff)/st(diff): 300012055 (1180) 300029944 ( 820) > > ut(diff)/st(diff): 300012055 ( 0) 300031944 (2000) > > ut(diff)/st(diff): 300012055 ( 0) 300033944 (2000) > > ut(diff)/st(diff): 300012055 ( 0) 300035944 (2000) > > ut(diff)/st(diff): 300012055 ( 0) 300037944 (2000) > > > > shows the problem even when sum_exec_runtime is not that big: 300000 secs. > > > > The new implementation of scale_stime() does the additional div64_u64_rem() > > in a loop but see the comment, as long it is used by cputime_adjust() this > > can happen only once. > > That only shows something after long long staring :/ There's no words on > what the output actually means or what would've been expected. > > Also, your example is incomplete; the below is a test for scale_stime(); > from this we can see that the division results in too large a number, > but, important for our use-case in cputime_adjust(), it is a step > function (due to loss in precision) and for every plateau we shift > runtime into the wrong bucket. But I'm still confused, since in the long run, it should still end up with a proportionally divided user/system, irrespective of some short term wobblies. So please, better articulate the problem.