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

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Message-ID: <48EDAD55.2000502@cn.fujitsu.com>
Date:	Thu, 09 Oct 2008 15:05:57 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	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

Andi Kleen wrote:
>> 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.
> 
> -Andi
> 
> 
> 

I thought the default value of NR_CPUS is 32.

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

Lai

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