From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wg0-x22e.google.com (mail-wg0-x22e.google.com [IPv6:2a00:1450:400c:c00::22e]) (using TLSv1 with cipher ECDHE-RSA-RC4-SHA (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "Google Internet Authority" (not verified)) by ozlabs.org (Postfix) with ESMTPS id 2660F2C00E0 for ; Thu, 25 Jul 2013 23:30:56 +1000 (EST) Received: by mail-wg0-f46.google.com with SMTP id k13so1622163wgh.13 for ; Thu, 25 Jul 2013 06:30:50 -0700 (PDT) Date: Thu, 25 Jul 2013 15:30:46 +0200 From: Frederic Weisbecker To: Preeti U Murthy Subject: Re: [RFC PATCH 4/5] cpuidle/ppc: CPU goes tickless if there are no arch-specific constraints Message-ID: <20130725133044.GA7400@somewhere> References: <20130725090016.12500.28888.stgit@preeti.in.ibm.com> <20130725090302.12500.42998.stgit@preeti.in.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20130725090302.12500.42998.stgit@preeti.in.ibm.com> Cc: rjw@sisk.pl, shangw@linux.vnet.ibm.com, arnd@arndb.de, linux-pm@vger.kernel.org, geoff@infradead.org, rostedt@goodmis.org, deepthi@linux.vnet.ibm.com, paul.gortmaker@windriver.com, paulus@samba.org, srivatsa.bhat@linux.vnet.ibm.com, schwidefsky@de.ibm.com, john.stultz@linaro.org, tglx@linutronix.de, paulmck@linux.vnet.ibm.com, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, chenhui.zhao@freescale.com List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Thu, Jul 25, 2013 at 02:33:02PM +0530, Preeti U Murthy wrote: > In the current design of timer offload framework, the broadcast cpu should > *not* go into tickless idle so as to avoid missed wakeups on CPUs in deep idle states. > > Since we prevent the CPUs entering deep idle states from programming the lapic of the > broadcast cpu for their respective next local events for reasons mentioned in > PATCH[3/5], the broadcast CPU checks if there are any CPUs to be woken up during > each of its timer interrupt programmed to its local events. > > With tickless idle, the broadcast CPU might not get a timer interrupt till after > many ticks which can result in missed wakeups on CPUs in deep idle states. By > disabling tickless idle, worst case, the tick_sched hrtimer will trigger a > timer interrupt every period to check for broadcast. > > However the current setup of tickless idle does not let us make the choice > of tickless on individual cpus. NOHZ_MODE_INACTIVE which disables tickless idle, > is a system wide setting. Hence resort to an arch specific call to check if a cpu > can go into tickless idle. Hi Preeti, I'm not exactly sure why you can't enter the broadcast CPU in dynticks idle mode. I read in the previous patch that's because in dynticks idle mode the broadcast CPU deactivates its lapic so it doesn't receive the IPI. But may be I misunderstood. Anyway that's not good for powersaving. Also when an arch wants to prevent a CPU from entering dynticks idle mode, it typically use arch_needs_cpu(). May be that could fit for you as well? Thanks.