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

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Date:	Wed, 26 Mar 2008 09:13:39 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Mike Travis <travis@....com>
CC:	Ingo Molnar <mingo@...e.hu>,
	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 wrote:
> 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.)
>   

It's a pity you can't take advantage of kasprintf to handle all this.

Hm, I would say that bitmap_scnprintf is a candidate for implementation 
as a printk format specifier so you could get away from needing a 
special function to print bitmaps...

Eh?  What's the difference between snprintf and scnprintf?

    J
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