linux-kernel - Re: [PATCH 0/3] x86: add cpus

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Date:	Wed, 09 Apr 2008 13:59:39 -0700
From:	Mike Travis <travis@....com>
To:	Greg KH <gregkh@...e.de>
CC:	Bert Wesarg <bert.wesarg@...glemail.com>,
	Ingo Molnar <mingo@...e.hu>, Paul Jackson <pj@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] x86: add cpus_scnprintf function v3

Greg KH wrote:
> On Wed, Apr 09, 2008 at 07:51:23PM +0200, Bert Wesarg wrote:
>> On Tue, Apr 8, 2008 at 8:43 PM, Mike Travis <travis@....com> wrote:
>>>   * Cleanup usages of cpumask_scprintf in the following files and add
>>>     another interface to use cpulist_scnprintf where appropriate.
>> On Mon, Apr 7, 2008 at 8:22 PM, Mike Travis <travis@....com> wrote:
>>>  Part of the change is readability, but also looking towards the future
>>>  of 16k/64k/??? # of cpus, the straight mask approach will overflow the
>>>  PAGE_SIZE buffer provided (though some pathological cases will overflow
>>>  the range method as well.)  So we'll need some advancement in the format
>>>  of the printout.
>> Btw, I think you can now push for a deprecation of the 'old' mask
>> attributes, with the justification you have given above. The other
>> possibility is to change sysfs to provide bigger attribute buffers
>> (CCed Greg for this).
> 
> Huh?
> 
> sysfs is "one value per file", if you are getting close to PAGE_SIZE in
> any sysfs file, then you are doing something very wrong.
> 
> What sysfs file currently is trying to output data this big?
> 
> thanks,
> 
> greg k-h

Hi Greg,

There's none at the moment.  The increase is coming from printing the
cpuset for various attributes, like cpus on a node, etc.  Since it uses
cpumask_scnprintf(), this prints a bit map representing a cpumask_t.

With the increase to 4096 cpus, this string is now 1152 bytes long.  The
next iteration will have 16384 cpus which will need 4608 bytes to fully
display, overflowing a standard page.  I've added alternate interfaces
that use cpulist_scnprintf() which has the advantage of collapsing the
bits into ranges.  This though can result in a much larger output size
if, for example only every other bit is set.

Btw, where does one value per file come from?  I see outputs like:

# cat /proc/self/stat
4313 (cat) R 4218 4313 4218 34816 4313 4194304 207 0 0 0 0 0 0 0 20 0 1 0 6802916 5672960 131 18446744073709551615 4194304 4212948 140735962676160 18446744073709551615 140499600349840 0 0 0 0 0 0 0 17 3 0 0 0 0 0

Thanks,
Mike
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