From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756473AbcBXLR4 (ORCPT ); Wed, 24 Feb 2016 06:17:56 -0500 Received: from www.linutronix.de ([62.245.132.108]:47116 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753877AbcBXLRh (ORCPT ); Wed, 24 Feb 2016 06:17:37 -0500 Date: Wed, 24 Feb 2016 12:16:19 +0100 (CET) From: Thomas Gleixner To: riel@redhat.com cc: linux-kernel@vger.kernel.org, fweisbec@gmail.com, mingo@kernel.org, luto@amacapital.net, peterz@infradead.org, clark@redhat.com, eric.dumazet@gmail.com Subject: Re: [PATCH 0/4 v6] sched,time: reduce nohz_full syscall overhead 40% In-Reply-To: <1455152907-18495-1-git-send-email-riel@redhat.com> Message-ID: References: <1455152907-18495-1-git-send-email-riel@redhat.com> User-Agent: Alpine 2.11 (DEB 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001,URIBL_BLOCKED=0.001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 10 Feb 2016, riel@redhat.com wrote: > (v6: make VIRT_CPU_ACCOUNTING_GEN jiffy granularity) > > Running with nohz_full introduces a fair amount of overhead. > Specifically, various things that are usually done from the > timer interrupt are now done at syscall, irq, and guest > entry and exit times. > > However, some of the code that is called every single time > has only ever worked at jiffy resolution. The code in > __acct_update_integrals was also doing some unnecessary > calculations. > > Getting rid of the unnecessary calculations, without > changing any of the functionality in __acct_update_integrals > gets us about an 11% win. > > Not calling the time statistics updating code more than > once per jiffy, like is done on housekeeping CPUs and on > all the CPUs of a non-nohz_full system, shaves off a > further 30%. Reviewed-by: Thomas Gleixner