CONFIG_DEBUG_SLAB_LEAK omits size-4096 and larger?

["J. Bruce Fields" <bfields@fieldses.org>]

I'm probably missing something fundamental--why doesn't
/proc/slab_allocators show any results for size-x where x >= 4096?

Someone's seeing a performance problem with the linux nfs server.  One
of the symptoms is the "size-4096" slab cache seems to be out of
control.  I assumed that meant that memory allocated by kmalloc() might
be leaking, so figured it might be interesting to turn on
CONFIG_DEBUG_SLAB_LEAK.  As far as I can tell what that does is list
kmalloc() callers in /proc/slab_allocators.  But that doesn't seem to be
showing any results for size-4096.  Can anyone provide a clue?
Thanks!

--b.

On Wed, Jun 11, 2008 at 02:46:13PM -0400, bfields wrote:
> On Tue, Jun 10, 2008 at 05:12:31PM -0500, Weathers, Norman R. wrote:
> >  
> > 
> > > -----Original Message-----
> > > From: J. Bruce Fields [mailto:bfields@fieldses.org] 
> > > Sent: Tuesday, June 10, 2008 12:16 PM
> > > To: Weathers, Norman R.
> > > Cc: linux-nfs@vger.kernel.org
> > > Subject: Re: Problems with large number of clients and reads
> > > 
> > > On Tue, Jun 10, 2008 at 09:30:18AM -0500, Weathers, Norman R. wrote:
> > > > Unfortunately, I cannot stop the clients (middle of long running
> > > > jobs).  I might be able to test this soon.  If I have the number of
> > > > threads high, yes I can reduce the number of threads and it 
> > > appears to
> > > > lower some of the memory, but even with as little as three threads,
> > > > the memory usage climbs very high, just not as high as if there are
> > > > say 8 threads.  When the memory usage climbs high, it can cause the
> > > > box to not respond over the network (ssh, rsh), and even be very
> > > > sluggish when I am connected over our serial console to the 
> > > server(s).
> > > > This same scenario has been happening with kernels that I have tried
> > > > from 2.6.22.x on to the 2.6.25 series.  The 2.6.25 series is
> > > > interesting in that I can push the same load from a box with the
> > > > 2.6.25 kernel and not have a load over .3 (with 3 threads), but with
> > > > the 2.6.22.x kernel, I have a load of over 3 when I hit the same
> > > > conditions.
> > > 
> > > OK, I think what we want to do is turn on 
> > > CONFIG_DEBUG_SLAB_LEAK.  I've
> > > never used it before, but it looks like it will report which functions
> > > are allocating from each slab cache, which may be exactly what we need
> > > to know.  So:
> > > 
> > > 	1. Install a kernel with both CONFIG_DEBUG_SLAB ("Debug slab
> > > 	memory allocations") and CONFIG_DEBUG_SLAB_LEAK ("Memory leak
> > > 	debugging") turned on.  They're both under the "kernel hacking"
> > > 	section of the kernel config.  (If you have a file
> > > 	/proc/slab_allocators, then you already have these turned on and
> > > 	you can skip this step.)
> > > 
> > > 	2. Do whatever you need to do to reproduce the problem.
> > > 
> > > 	3. Get a copy of /proc/slabinfo and /proc/slab_allocators.
> > > 
> > > Then we can take a look at that and see if it sheds any light.
> > 
> > 
> > I have taken several snapshots of the /proc/slab_allocators and
> > /proc/slabinfo as requested, but since there is a lot of info in them,
> > and I didn't think anyone wanted to go cross-eyed reading the data in an
> > email, I have them up on a website:
> > 
> > http://shashi-weathers.net/linux/cluster/NFS/
> 
> Excellent.
> 
> > 
> > The order of data collection is:
> > 
> > slab_allocators_bad1.txt and corresponding slabinfo
> > slab_allocators_after_bad1.txt and corresponding slabinfo
> > slab_allocators_16_threads.txt and corresponding slabinfo
> > slab_allocators_16_threads_1.txt and corresponding slabinfo
> > slab_allocators_32_threads.txt and corresponding slabinfo
> > slab_allocators_really_bad.txt and corresponding slabinfo.
> > 
> > 
> > You will have to forgive my ignorance at this point, but I was looking
> > through the slabinfo and slab_allocators, and noticed that size-4096
> > does not show up in slab_allocators... I hope that is by design.  You
> > can see it growing into the gigabytes in the slabinfo files....
> 
> Argh. OK, I don't understand well enough how this works.  Time to ask
> someone, I guess....
> 
> --b.
> 
> > 
> > 
> > 
> > > 
> > > I think that debugging will hurt the server performance, so you won't
> > > want to keep it turned on all the time.
> > > 
> > > > 
> > > > Also, this is all with the SLAB cache option.  SLUB crashes 
> > > everytime
> > > > I use it under heavy load.
> > > 
> > > Have you reported the SLUB bugs to lkml?
> > 
> > No, I haven't yet.  I didn't know for sure if I was doing something
> > wrong, or if SLUB was the problem there.  Since the failures, I had gone
> > back to using SLAB anyway, so ....  I probably should...
> > 
> > > 
> > > --b.
> > > 
> > 
> > 
> > Norman Weathers