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SubjectRe: [PATCH 06/10] x86: reduce memory and stack usage in intel_cacheinfo
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.)

Thanks,
Mike


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