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SubjectRe: [PATCH 06/10] x86: reduce memory and stack usage in intel_cacheinfo
Mike Travis wrote:
> Ingo Molnar wrote:
>
>> * Mike Travis <travis@sgi.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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