From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756037AbcA3Drp (ORCPT ); Fri, 29 Jan 2016 22:47:45 -0500 Received: from mx1.redhat.com ([209.132.183.28]:41316 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753836AbcA3Drn (ORCPT ); Fri, 29 Jan 2016 22:47:43 -0500 Subject: Re: [PATCH 1/2] sched,time: remove pointless divides from __acct_update_integrals To: Frederic Weisbecker , Peter Zijlstra References: <1454106180-20918-1-git-send-email-riel@redhat.com> <1454106180-20918-2-git-send-email-riel@redhat.com> <20160129231018.GK6357@twins.programming.kicks-ass.net> <20160130033632.GA14020@lerouge> Cc: linux-kernel@vger.kernel.org, tglx@linutronix.de, mingo@kernel.org, luto@amacapital.net From: Rik van Riel Message-ID: <56AC325D.7040100@redhat.com> Date: Fri, 29 Jan 2016 22:47:41 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 MIME-Version: 1.0 In-Reply-To: <20160130033632.GA14020@lerouge> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/29/2016 10:36 PM, Frederic Weisbecker wrote: > On Sat, Jan 30, 2016 at 12:10:18AM +0100, Peter Zijlstra wrote: >> On Fri, Jan 29, 2016 at 05:22:59PM -0500, riel@redhat.com wrote: >>> From: Rik van Riel >>> >>> When running a microbenchmark calling an invalid syscall number >>> in a loop, on a nohz_full CPU, we spend a full 9% of our CPU >>> time in __acct_update_integrals. >>> >>> This function converts cputime_t to jiffies, to a timeval, only to >>> convert the timeval back to microseconds before discarding it. >>> >>> This patch leaves __acct_update_integrals functionally equivalent, >>> but speeds things up by about 11%, with 10 million calls to an >>> invalid syscall number dropping from 3.7 to 3.3 seconds. >> >> WTH is this taskstat crap anyway? Who uses it and can't we kill it? > > I have no idea what it's used for, it seems to be related to taskstats > over netlink. I'm not even sure if it's actually used. There don't seem > to be a runtime offcase and I bet distros enable it. So that stuff does > some work every millisecond on millions of machines while it probably > has very few users. > > SGI introduced it in 2006 and it seems that their last contribution there is > in 2008. The rest is kernel maintainance and fixes. > > If there are still users of it, then at least we should disable it on runtime by > default. With all the non-power-of-2 divides removed, __acct_update_integrals disappears from the profile, even running many times per millisecond. At that point, native_sched_clock takes over the top of the profile, and the only way I can think of getting rid of that one is making sure it is not called twice for every syscall, irq, and kvm guest entry/exit. -- All rights reversed