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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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