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

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Date:	Wed, 26 Mar 2008 09:27:04 -0700
From:	Mike Travis <travis@....com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
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>
Subject: Re: [PATCH 06/10] x86: reduce memory and stack usage in	intel_cacheinfo

Jeremy Fitzhardinge wrote:
> 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...

Hmm, I hadn't thought of that.  There is commonly a format spec called
%b for diags, etc. to print bit strings.  Maybe something like:

	"... %*b ...", nr_cpu_ids, ptr_to_bitmap

where the length arg is rounded up to 32 or 64 bits...? 

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

Good question... I'll have to ask the cpumask person. ;-)
> 
>    J

Thanks!
Mike
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