linux-kernel - Re: [PATCH 2/2] x86: modify show_shared_cpu_map in intel

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Date:	Fri, 28 Mar 2008 15:54:43 +0100
From:	"Bert Wesarg" <bert.wesarg@...glemail.com>
To:	"Mike Travis" <travis@....com>
Cc:	"Ingo Molnar" <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86: modify show_shared_cpu_map in intel_cacheinfo

On Fri, Mar 28, 2008 at 3:40 PM, Mike Travis <travis@....com> wrote:
>
> Bert Wesarg wrote:
>  > On Fri, Mar 28, 2008 at 12:16 AM, Mike Travis <travis@....com> wrote:
>  >> Used cpulist_scnprintf to print cpus on a leaf instead of requiring
>  >>  a new "cpumask_scnprintf_len" function to determine the size of
>  >>  the temporary buffer.  cpulist_scnprintf can be used to print directly
>  >>  to the output buffer, eliminating the need for the temporary buffer.
>  >>
>  >>  Based on:
>  >>         git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git
>  >>         git://git.kernel.org/pub/scm/linux/kernel/git/x86/linux-2.6-x86.git
>  >>
>  >>  Signed-off-by: Mike Travis <travis@....com>
>  >>  ---
>  >>   arch/x86/kernel/cpu/intel_cacheinfo.c |   25 +++++++++++++++++--------
>  >>   1 file changed, 17 insertions(+), 8 deletions(-)
>  >>
>  >>  --- linux.trees.git.orig/arch/x86/kernel/cpu/intel_cacheinfo.c
>  >>  +++ linux.trees.git/arch/x86/kernel/cpu/intel_cacheinfo.c
>  >>  @@ -593,14 +593,23 @@ static ssize_t show_size(struct _cpuid4_
>  >>
>  >>   static ssize_t show_shared_cpu_map(struct _cpuid4_info *this_leaf, char *buf)
>  >>   {
>  >>  -       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);
>  >>  +       /*
>  >>  +        * 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.
>  > So this breaks user space?
>  >
>  > Bert
>
>  Potentially, yes.  But I suspect with 4096 cpus, user scripts will have
>  to change anyways.  Currently it is represented as sets of 32 bit mask
>  outputs with comma separators, so 1152 characters would be output.
But you can declare it as a programming error on user space side. If
you change the format, the brown-paper-bag is yours.

>
>  Is there a special notice I should provide to announce this change?
I hope so. At least lwn.net has an API changes site:

http://lwn.net/Articles/2.6-kernel-api/

I also looked into MAINTAINERS, but it seems there is no official API
'maintainer'.

>
>  (And this output does conform with other syntax for printing and parsing
>  strings of bits.)
Aren't the most cpumaps (like cpu/cpu*/topology/*_siblings or
node/node*/cpumap) bitmasks?

Bert
>
>  Thanks,
>  Mike
>
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