Re: 2.6.39-rc4+: Kernel leaking memory during FS scanning, regression?

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Apr 25, 2011 at 07:00:32PM +0200, Bruno Prémont wrote:
> On Mon, 25 April 2011 Linus Torvalds wrote:
> > 2011/4/25 Bruno Prémont <bonbons@linux-vserver.org>:
> > >
> > > kmemleak reports 86681 new leaks between shortly after boot and -2 state.
> > > (and 2348 additional ones between -2 and -4).
> > 
> > I wouldn't necessarily trust kmemleak with the whole RCU-freeing
> > thing. In your slubinfo reports, the kmemleak data itself also tends
> > to overwhelm everything else - none of it looks unreasonable per se.
> > 
> > That said, you clearly have a *lot* of filp entries. I wouldn't
> > consider it unreasonable, though, because depending on load those may
> > well be fine. Perhaps you really do have some application(s) that hold
> > thousands of files open. The default file limit is 1024 (I think), but
> > you can raise it, and some programs do end up opening tens of
> > thousands of files for filesystem scanning purposes.
> > 
> > That said, I would suggest simply trying a saner kernel configuration,
> > and seeing if that makes a difference:
> > 
> > > Yes, it's uni-processor system, so SMP=n.
> > > TINY_RCU=y, PREEMPT_VOLUNTARY=y (whole /proc/config.gz attached keeping
> > > compression)
> > 
> > I'm not at all certain that TINY_RCU is appropriate for
> > general-purpose loads. I'd call it more of a "embedded low-performance
> > option".
> 
> Well, TINY_RCU is the only option when doing PREEMPT_VOLUNTARY on
> SMP=n...

You can either set SMP=y and NR_CPUS=1 or you can handed-edit
init/Kconfig to remove the dependency on SMP.  Just change the

	depends on !PREEMPT && SMP

to:

	depends on !PREEMPT

This will work fine, especially for experimental purposes.

> > The _real_ RCU implementation ("tree rcu") forces quiescent states
> > every few jiffies and has logic to handle "I've got tons of RCU
> > events, I really need to start handling them now". All of which I
> > think tiny-rcu lacks.
> 
> Going to try it out (will take some time to compile), kmemleak disabled.
> 
> > So right now I suspect that you have a situation where you just have a
> > simple load that just ends up never triggering any RCU cleanup, and
> > the tiny-rcu thing just keeps on gathering events and delays freeing
> > stuff almost arbitrarily long.
> 
> I hope tiny-rcu is not that broken... as it would mean driving any
> PREEMPT_NONE or PREEMPT_VOLUNTARY system out of memory when compiling
> packages (and probably also just unpacking larger tarballs or running
> things like du).

If it is broken, I will fix it.  ;-)

							Thanx, Paul

> And with system doing nothing (except monitoring itself) memory usage
> goes increasing all the time until it starves (well it seems to keep
> ~20M free, pushing processes it can to swap). Config is just being
> make oldconfig from working 2.6.38 kernel (answering default for new
> options)
> 
> Memory usage evolution graph in first message of this thread:
> http://thread.gmane.org/gmane.linux.kernel.mm/61909/focus=1130480
> 
> Attached graph matching numbers of previous mail. (dropping caches was at
> 17:55, system idle since then)
> 
> Bruno
> 
> 
> > So try CONFIG_PREEMPT and CONFIG_TREE_PREEMPT_RCU to see if the
> > behavior goes away. That would confirm the "it's just tinyrcu being
> > too dang stupid" hypothesis.
> > 
> >                      Linus


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