From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755710Ab3LRVtg (ORCPT ); Wed, 18 Dec 2013 16:49:36 -0500 Received: from e32.co.us.ibm.com ([32.97.110.150]:34700 "EHLO e32.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754003Ab3LRVte (ORCPT ); Wed, 18 Dec 2013 16:49:34 -0500 Date: Wed, 18 Dec 2013 13:49:30 -0800 From: "Paul E. McKenney" To: Andy Lutomirski Cc: Frederic Weisbecker , Alex Shi , LKML , Thomas Gleixner , Ingo Molnar , Peter Zijlstra , Steven Rostedt , John Stultz , Kevin Hilman Subject: Re: [RFC PATCH 00/13] nohz: Use sysidle detection to let the timekeeper sleep Message-ID: <20131218214930.GU19211@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1387320692-28460-1-git-send-email-fweisbec@gmail.com> <52B102BB.1050907@linaro.org> <20131218174355.GA19788@localhost.localdomain> <52B213D1.1000104@mit.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <52B213D1.1000104@mit.edu> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13121821-0928-0000-0000-000004D393AC Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 18, 2013 at 01:29:53PM -0800, Andy Lutomirski wrote: > On 12/18/2013 09:43 AM, Frederic Weisbecker wrote: > > On Wed, Dec 18, 2013 at 10:04:43AM +0800, Alex Shi wrote: > >> On 12/18/2013 06:51 AM, Frederic Weisbecker wrote: > >>> So this is what this series brings, more details following: > >>> > >>> * Some code, naming and whitespace cleanups > >>> > >>> * Allow all CPUs outside the nohz_full range to handle the timekeeping > >>> duty, not just CPU 0. Balancing the timekeeping duty should improve > >>> powersavings. > >> > >> If the system just has one nohz_full cpu running, it will need another > >> cpu to do timerkeeper job. Then the system roughly needs 2 cpu living. > >> From powersaving POV, that is not good compare to normal nohz idle. > > > > Sure, but everything has a tradeoff :) > > > > We could theoretically run with the timekeeper purely idle if the other > > CPU in full dynticks mode runs in userspace for a long while and seldom > > do syscalls and faults. Timekeeping could be updated on kernel/user > > boundaries in this case without much impact on performances. > > > > But then there is one strict condition for that: it can't read the timeofday > > through the vdso but only through a syscall. > > Where's your ambition? :) > > If the vdso timing functions could see that it's been too long since a > real timekeeping update, they could fall back to a syscall. Otherwise, > they could using rdtsc or whatever is in use. One objection to that approach in the past has been that it injects avoidable latency into the worker CPUs. I suppose that you could argue that the cache misses due to a timekeeping-CPU update are not free, but then again, the syscall is likely to also incur a few cache misses as well. I bet that the timekeeping-CPU approach wins, but it would be cool to see you prove me wrong. Thanx, Paul