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

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Date:	Wed, 26 Mar 2008 08:41:23 -0700
From:	Mike Travis <travis@....com>
To:	Ingo Molnar <mingo@...e.hu>
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

Ingo Molnar wrote:
> * Mike Travis <travis@....com> wrote:
> 
>> * Change the following static arrays sized by NR_CPUS to
>>   per_cpu data variables:
>>
>> 	_cpuid4_info *cpuid4_info[NR_CPUS];
>> 	_index_kobject *index_kobject[NR_CPUS];
>> 	kobject * cache_kobject[NR_CPUS];
>>
>> * Remove the local NR_CPUS array with a kmalloc'd region in
>>   show_shared_cpu_map().
> 
> thanks Travis, i've applied this to x86.git.
> 
> one observation:
> 
>>  static ssize_t show_shared_cpu_map(struct _cpuid4_info *this_leaf, char *buf)
>>  {
>> -	char mask_str[NR_CPUS];
>> -	cpumask_scnprintf(mask_str, NR_CPUS, this_leaf->shared_cpu_map);
>> -	return sprintf(buf, "%s\n", mask_str);
>> +	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.)

Thanks,
Mike
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