linux-kernel - Re: [PATCH] Don't export sched_mc_power

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Date:	Mon, 1 Dec 2008 08:44:47 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Mahesh Jagannath Salgaonkar <mahesh@...ux.vnet.ibm.com>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Suresh B Siddha <suresh.b.siddha@...el.com>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	svaidy@...ux.vnet.ibm.com
Subject: Re: [PATCH] Don't export sched_mc_power_savings in laptops


* Mahesh Jagannath Salgaonkar <mahesh@...ux.vnet.ibm.com> wrote:

> Ingo Molnar wrote:
>> * Mahesh Salgaonkar <mahesh@...ux.vnet.ibm.com> wrote:
>>
>>   
>>>         Fix to prevent sched_mc_power_saving from being exported through sysfs
>>>         for multicore single socket (Laptop).
>>>             CPU core map of the boot cpu should be equal to possible 
>>> number
>>>         of cpus for single socket system.
>>>             This fix has been developed at FOSS.in kernel workout.
>>>             Signed-off-by: Mahesh Salgaonkar 
>>> <mahesh@...ux.vnet.ibm.com>
>>>
>>> diff --git a/arch/x86/include/asm/topology.h b/arch/x86/include/asm/topology.h
>>> index 4850e4b..4adc830 100644
>>> --- a/arch/x86/include/asm/topology.h
>>> +++ b/arch/x86/include/asm/topology.h
>>> @@ -239,7 +239,7 @@ struct pci_bus;
>>>  void set_pci_bus_resources_arch_default(struct pci_bus *b);
>>>   #ifdef CONFIG_SMP
>>> -#define mc_capable()			(boot_cpu_data.x86_max_cores > 1)
>>> +#define mc_capable()	(cpus_weight(per_cpu(cpu_core_map,0)) != nr_cpu_ids)
>>>     
>>
>> hm, dunno. sched_mc_power_savings should have no effect on 
>> single-socket systems, right? So the knob should have no effect.
>>
>> 	Ingo
>>   
> Hi Ingo,
>
> Yes, it will not have any effect but it enables additional code path 
> leading to an overhead in case of single-socket system. This overhead 
> is only if "sched_mc_power_savings = 1", however we are still leaving 
> it to zero (default) and preventing user space from turning it on by 
> mistake.

okay, makes sense - i've applied it to tip/sched/urgent (for v2.6.28 
merge), thanks Mahesh!

	Ingo
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