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

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Message-Id: <20081009161232.DEDC.KOSAKI.MOTOHIRO@jp.fujitsu.com>
Date:	Thu,  9 Oct 2008 16:14:26 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	kosaki.motohiro@...fujitsu.com, 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

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.


> 
> 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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