linux-kernel - Re: [PATCH 06/10] x86: reduce memory and stack usage in intel

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Date:	Wed, 26 Mar 2008 18:12:41 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Mike Travis <travis@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...e.de>
Subject: Re: [PATCH 06/10] x86: reduce memory and stack usage in
	intel_cacheinfo


* Mike Travis <travis@....com> wrote:

> >> +	int n = 0;
> >> +	int len = cpumask_scnprintf_len(nr_cpu_ids);
> >> +	char *mask_str = kmalloc(len, GFP_KERNEL);
> >> +
> >> +	if (mask_str) {
> >> +		cpumask_scnprintf(mask_str, len, this_leaf->shared_cpu_map);
> >> +		n = sprintf(buf, "%s\n", mask_str);
> >> +		kfree(mask_str);
> >> +	}
> >> +	return n;
> > 
> > the other changes look good, but this one looks a bit ugly and complex. 
> > We basically want to sprintf shared_cpu_map into 'buf', but we do that 
> > by first allocating a temporary buffer, print a string into it, then 
> > print that string into another buffer ...
> > 
> > this very much smells like an API bug in cpumask_scnprintf() - why dont 
> > you create a cpumask_scnprintf_ptr() API that takes a pointer to a 
> > cpumask? Then this change would become a trivial and much more readable:
> > 
> >  -	char mask_str[NR_CPUS];
> >  -	cpumask_scnprintf(mask_str, NR_CPUS, this_leaf->shared_cpu_map);
> >  -	return sprintf(buf, "%s\n", mask_str);
> >  +	return cpumask_scnprintf_ptr(buf, NR_CPUS, &this_leaf->shared_cpu_map);
> > 
> > 	Ingo
> 
> The main goal was to avoid allocating 4096 bytes when only 32 would do 
> (characters needed to represent nr_cpu_ids cpus instead of NR_CPUS 
> cpus.) But I'll look at cleaning it up a bit more.  It wouldn't have 
> to be a function if CHUNKSZ in cpumask_scnprintf() were visible (or a 
> non-changeable constant.)

well, do we care about allocating 4096 bytes, as long as we also free 
it? It's not like we need to clear all the bytes or something. Am i 
missing something here?

	Ingo
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