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SubjectRe: [PATCH 06/10] x86: reduce memory and stack usage in intel_cacheinfo
Jeremy Fitzhardinge wrote:
> 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...

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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