From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755258Ab3GAT40 (ORCPT ); Mon, 1 Jul 2013 15:56:26 -0400 Received: from e34.co.us.ibm.com ([32.97.110.152]:45317 "EHLO e34.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755146Ab3GAT4Z (ORCPT ); Mon, 1 Jul 2013 15:56:25 -0400 Date: Mon, 1 Jul 2013 12:56:03 -0700 From: "Paul E. McKenney" To: Christoph Lameter Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu Subject: Re: [PATCH RFC nohz_full 0/7] v2 Provide infrastructure for full-system idle Message-ID: <20130701195603.GT3773@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20130628200949.GA17458@linux.vnet.ibm.com> <0000013f9bc32a5e-258db1be-840b-4b2f-9b3f-e1e3d5253cfc-000000@email.amazonses.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0000013f9bc32a5e-258db1be-840b-4b2f-9b3f-e1e3d5253cfc-000000@email.amazonses.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: No X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13070119-2876-0000-0000-00000A6E8B7D Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jul 01, 2013 at 07:43:47PM +0000, Christoph Lameter wrote: > On Fri, 28 Jun 2013, Paul E. McKenney wrote: > > > Unfortunately, timekeeping CPU continues taking scheduling-clock > > interrupts even when all other CPUs are completely idle, which is > > not so good for energy efficiency and battery lifetime. Clearly, it > > would be good to turn off the timekeeping CPU's scheduling-clock tick > > when all CPUs are completely idle. This is conceptually simple, but > > we also need good performance and scalability on large systems, which > > rules out implementations based on frequently updated global counts of > > non-idle CPUs as well as implementations that frequently scan all CPUs. > > Nevertheless, we need a single global indicator in order to keep the > > overhead of checking acceptably low. > > Can we turn off timekeeping when no cpu needs time in adaptive mode? > Setting breakpoints in the VDSO could force timekeeping on again whenever > something needs time. Would this not be simpler? Might be. But what causes the breakpoints to be set on a system where there is one CPU-bound nohz_full user-mode task with all other CPUs idle? Or are you suggesting taking a breakpoint trap on each timekeeping access to VDSO? Thanx, Paul