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Date: Thu, 5 Jun 2014 14:23:36 +1000
From: Paul Mackerras <paulus@samba.org>
To: Preeti U Murthy <preeti@linux.vnet.ibm.com>
Subject: Re: [PATCH 2/6] powerpc, powernv, CPU hotplug: Put offline CPUs in
 Fast-Sleep instead of Nap
Message-ID: <20140605042336.GA3199@drongo>
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Cc: svaidyan@in.ibm.com, linuxppc-dev@lists.ozlabs.org,
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On Wed, May 28, 2014 at 10:08:56AM +0530, Preeti U Murthy wrote:
> From: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
> 
> The offline cpus are put to fast sleep if the idle state is discovered in the
> device tree. This is to gain maximum powersavings in the offline state.

...

>  	while (!generic_check_cpu_restart(cpu)) {
>  		ppc64_runlatch_off();
> -		power7_nap();
> +
> +		/* If sleep is supported, go to sleep, instead of nap */
> +		if (idle_states & IDLE_USE_SLEEP)
> +			power7_sleep();
> +		else
> +			power7_nap();
> +
>  		ppc64_runlatch_on();
>  		if (!generic_check_cpu_restart(cpu)) {
>  			DBG("CPU%d Unexpected exit while offline !\n", cpu);

What is the latency for waking up from fast sleep state?  I'm concerned
this will increase the latency for entering KVM guests.

Paul.