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

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Message-ID: <49338933.1030401@linux.vnet.ibm.com>
Date:	Mon, 01 Dec 2008 12:20:27 +0530
From:	Mahesh Jagannath Salgaonkar <mahesh@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
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

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.

Thanks,
-Mahesh.


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