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SubjectRe: [PATCH 06/10] x86: reduce memory and stack usage in intel_cacheinfo
Ingo Molnar wrote:
> * H. Peter Anvin <hpa@zytor.com> wrote:
>
>> Ingo Molnar wrote:
>>>> 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.)
>>> well, do we care about allocating 4096 bytes, as long as we also free it?
>>> It's not like we need to clear all the bytes or something. Am i missing
>>> something here?
>> Well, 32 bytes fits on the stack, whereas 4096 bytes requires
>> allocating a page -- which means either taking the risk of failing or
>> blocking. Of course, we're doing this for output, which has the same
>> issue.
>
> hm, i thought this was all implemented via dynamic allocation already,
> within the cpumask_scnprintf function. But i see it doesnt do it - i
> guess a new call could be introduced, cpumask_scnprintf_ptr() which
> passes in a cpumask pointer and does dynamic allocation itself?
>
> Ingo

Here's a snippet of the new patch. This works fine (I think) for
cpus on a leaf. The sched_debug_one problem should work the same way,
hopefully ;-)

[sorry, cut and pasted so no tabs]

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);
+ /*
+ * cpulist_scnprintf() has the advantage of compressing
+ * consecutive cpu numbers into a single range which seems
+ * appropriate for cpus on a leaf. This will change what is
+ * output so scripts that process the output will have to change.
+ * The good news is that the output format is compatible
+ * with cpulist_parse() [bitmap_parselist()].
+ *
+ * Have to guess at output buffer size... 128 seems reasonable
+ * to represent all cpus on a leaf in the worst case, like
+ * if all cpus are non-consecutive and large numbers.
+ */
+ return cpulist_scnprintf(buf, 128, this_leaf->shared_cpu_map);
}

Thanks,
Mike


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