From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752610Ab3FZOvd (ORCPT ); Wed, 26 Jun 2013 10:51:33 -0400 Received: from merlin.infradead.org ([205.233.59.134]:58975 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752553Ab3FZOvb (ORCPT ); Wed, 26 Jun 2013 10:51:31 -0400 Date: Wed, 26 Jun 2013 14:20:22 +0200 From: Peter Zijlstra To: "Paul E. McKenney" 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, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu Subject: Re: [PATCH RFC nohz_full 0/8] Provide infrastructure for full-system idle Message-ID: <20130626122022.GI28407@twins.programming.kicks-ass.net> References: <20130625213721.GA19452@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130625213721.GA19452@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 25, 2013 at 02:37:21PM -0700, Paul E. McKenney wrote: > Whenever there is at least one non-idle CPU, it is necessary to > periodically update timekeeping information. Before NO_HZ_FULL, this > updating was carried out by the scheduling-clock tick, which ran on > every non-idle CPU. With the advent of NO_HZ_FULL, it is possible > to have non-idle CPUs that are not receiving scheduling-clock ticks. > This possibility is handled by assigning a timekeeping CPU that continues > taking scheduling-clock ticks. > > 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. > > The chosen approach is to enforce hysteresis on the non-idle to > full-system-idle transition, with the amount of hysteresis increasing > linearly with the number of CPUs, thus keeping contention acceptably low. > This approach piggybacks on RCU's existing force-quiescent-state scanning > of idle CPUs, which has the advantage of avoiding the scan entirely on > busy systems that have high levels of multiprogramming. This scan > take per-CPU idleness information and feeds it into a state machine > that applies the level of hysteresis required to arrive at a single > full-system-idle indicator. > > Note that this version pays attention to CPUs that have taken an NMI > from idle. It is not clear to me that NMI handlers can safely access > the time on a system that is long-term idle. Unless someone tells me > that it is somehow safe to access time from an NMI from idle, I will > remove NMI support in the next version. Using perf it is 'possible' to come near; we use local_clock() from NMI context. It will do a TSC read. On systems where the TSC is usable we'll end up with a sane timestamp; on systems where we need the whole kernel/sched/clock.c song and dance routine we'll return a stable time-stamp when called from long idle. I don't think there's anything we can do better there.