linux-kernel - Re: [PATCH 2/2] SLUB: Disable debugging if it increases the minimum page order

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Date:	Thu, 11 Jun 2009 09:39:35 -0500
From:	Larry Finger <Larry.Finger@...inger.net>
To:	Pekka Enberg <penberg@...helsinki.fi>
CC:	Christoph Lameter <cl@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, mel@....ul.ie
Subject: Re: [PATCH 2/2] SLUB: Disable debugging if it increases the minimum
 page order

Pekka Enberg wrote:
> Hi Christoph,
> 
> On Thu, 2009-06-11 at 09:43 -0400, Christoph Lameter wrote:
>> We have had that with SLAB. NO! This leads to the situation that some
>> slabs have debug on and some have not. You just do not know which.
> 
> I do see your point but surely we don't want to use order 1 allocations
> in the fall-back case for kmalloc-4096? Couldn't we just add a printk
> saying that debug was disabled for the cache? After all, my patch is
> much better than what SLAB does.
> 
> On Thu, 2009-06-11 at 09:43 -0400, Christoph Lameter wrote:
>> Note that CONFIG_SLUB_DEBUG only enables the code to debug a slab. It does
>> not enable debugging for each slab. CONFIG_SLUB_DEBUG_ON does that.
> 
> True. Larry, do you have CONFIG_SLUB_DEBUG_ON enabled or are you passing
> SLUB debugging options to the kernel?

It is enabled in the kernel. My SLUB options in .config are:

finger@...rylap:~> grep SLUB .config
CONFIG_SLUB_DEBUG=y
CONFIG_SLUB=y
CONFIG_SLUB_DEBUG_ON=y
# CONFIG_SLUB_STATS is not set

Would having STATS enabled help?

For a bug that hits infrequently, determining that it is fixed is
difficult. That said, my system has been up about 22.5 hours during
which I have tried to force the failure. I see the fragmentation of
memory vary widely as shown below:

finger@...rylap:~> date ; cat /proc/buddyinfo
Wed Jun 10 20:15:34 CDT 2009
Node 0, zone      DMA      4      3      5      2      4      1      2
     0      1      0      0
Node 0, zone    DMA32   5920  11678   2245    369    117     21      2
     1      0      0      0
finger@...rylap:~> date ; cat /proc/buddyinfo
Wed Jun 10 23:32:39 CDT 2009
Node 0, zone      DMA      4      3      5      2      4      1      2
     0      1      0      0
Node 0, zone    DMA32   2605   4140   3804      7      0      1      1
     1      0      0      0
finger@...rylap:~> date ; cat /proc/buddyinfo
Thu Jun 11 09:28:06 CDT 2009
Node 0, zone      DMA      4      3      5      2      4      1      2
     0      1      0      0
Node 0, zone    DMA32   2231    429   2726     54      1      0      1
     1      0      0      0
finger@...rylap:~> cat /proc/uptime
80678.11 78.54

The latest value of the number of O(1) fragments is about as low as I
have seen.

Larry
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