linux-kernel - Re: [PATCH] rudimentary tracing for Classic RCU

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Message-ID: <48EDB22D.6050908@cn.fujitsu.com>
Date:	Thu, 09 Oct 2008 15:26:37 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
CC:	Andi Kleen <andi@...stfloor.org>, paulmck@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, mingo@...e.hu, rjw@...k.pl,
	dipankar@...ibm.com, tglx@...uxtronix.de
Subject: Re: [PATCH] rudimentary tracing for Classic RCU

KOSAKI Motohiro wrote:
> Hi Lai-san,
> 
>>>> In this case, why not define it as:
>>>> static char buf[20*NR_CPUS + 100];
>>> Actually you should near never use NR_CPUS now but always num_possible_cpus()
>>> (or even num_online_cpus()) Using NR_CPUS can lead to extreme waste
>>> of memory on kernels which are compiled for 4096 CPUs for example.
>>>
>>> And with num_possible_cpus() kmalloc is needed.
>> I thought the default value of NR_CPUS is 32.
> 
> it's pointless.
> Almost distribution use _very_ large NR_CPUS (likes 4096).
> So, We should concern large NR_CPUS.

Thanks.

So my advice is to use seq_file.

> 
> 
>> if NR_CPUS is too large, I think using seq_file is good idea,
>> and rcuclassic_trace_mutex is not need too.
> 
> 
> 
> 
> 


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