linux-kernel - Re: cpuidle asymmetry (was Re: [RFC PATCH V4 5/5] cpuidle: cpuidle driver for apm)

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Message-ID: <20110403161856.GB3376@in.ibm.com>
Date:	Sun, 3 Apr 2011 21:48:56 +0530
From:	Dipankar Sarma <dipankar@...ibm.com>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	Len Brown <lenb@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Trinabh Gupta <trinabh@...ux.vnet.ibm.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	suresh.b.siddha@...el.com, benh@...nel.crashing.org,
	venki@...gle.com, ak@...ux.intel.com, linux-kernel@...r.kernel.org,
	xen-devel@...ts.xensource.com
Subject: Re: cpuidle asymmetry (was Re: [RFC PATCH V4 5/5] cpuidle: cpuidle
 driver for apm)

On Fri, Apr 01, 2011 at 07:38:23AM -0700, Arjan van de Ven wrote:
> On 4/1/2011 1:15 AM, Dipankar Sarma wrote:
> >On Fri, Apr 01, 2011 at 12:09:25AM -0400, Len Brown wrote:
> >>>>Moorestown is already an example of an asymmetric system,
> >>>>since its deepest c-state is available on cpu0, but not on cpu1.
> >>>>So it needs different tables for each cpu.
> >>>wtf are these hardware guys smoking and how the heck are we supposed to
> >>>schedule on such a machine? Prefer to keep cpu1 busy while idling cpu0?
> >>they are smoking micro-amps:-)
> >>
> >>S0i3 on cpu0 can be entered only after cpu1 is already off-line,
> >>among other system hardware dependencies...
> >>
> >>So it makes no sense to export S0i3 as a c-state on cpu1.
> >>
> >>When cpu1 is online, the scheduler treats it as a normal SMP.
> >Isn't S0i3 a "system" state, as opposed to cpu state ?
> 
> it's misnamed. it's a C state to the OS.

I understand that it has been implemented as a C-state from this -
http://build.meego.com/package/view_file?file=linux-2.6.37-mrst-s0i3.patch&package=kernel-adaptation-mrst&project=home%3Adliu9&srcmd5=a0929a2863150f5c8454507d6cd8f09d

The key question is this -

+int mrst_check_state_availability(struct cpuidle_device *dev)  
+{  
+   int cpu = smp_processor_id();  
+ 
+   /*  
+    * If there is another CPU running, the GPU is active,  
+    * the PMU is uninitialized, or there is a still-unprocessed  
+    * PMU command, we cannot enter S0i3.  
+    */  
+   if (!pmu_reg || !cpumask_equal(cpu_online_mask, cpumask_of(cpu)) ||  
+       s0i3_pmu_command_pending)  
+       dev->states[5].flags |= CPUIDLE_FLAG_IGNORE;  
+   else  
+       dev->states[5].flags &= ~CPUIDLE_FLAG_IGNORE;  

Is this really asymetric ? Or is it that if the other
cpu(s) are in C6, the chip can enter S0i3 ? If that is the case,
then isn't this equivalent to all the cpus in the chip
entering S0i3 and thus symmetrical ? Also, if going to S0i3
relies on other cpus offlined, then we don't need to
worry about asymetry from the scheduler/cpuidle point of
view, no ?

Thanks
Dipankar
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