linux-kernel - Re: [RFC PATCH 4/5] cpuidle/ppc: CPU goes tickless if there are no arch-specific constraints

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Message-ID: <20130725133044.GA7400@somewhere>
Date:	Thu, 25 Jul 2013 15:30:46 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	benh@...nel.crashing.org, paul.gortmaker@...driver.com,
	paulus@...ba.org, shangw@...ux.vnet.ibm.com,
	galak@...nel.crashing.org, paulmck@...ux.vnet.ibm.com,
	michael@...erman.id.au, arnd@...db.de, linux-pm@...r.kernel.org,
	rostedt@...dmis.org, rjw@...k.pl, john.stultz@...aro.org,
	tglx@...utronix.de, chenhui.zhao@...escale.com,
	deepthi@...ux.vnet.ibm.com, geoff@...radead.org,
	linux-kernel@...r.kernel.org, srivatsa.bhat@...ux.vnet.ibm.com,
	schwidefsky@...ibm.com, svaidy@...ux.vnet.ibm.com,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH 4/5] cpuidle/ppc: CPU goes tickless if there are no
 arch-specific constraints

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.
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