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

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Date:	Tue, 26 Apr 2011 04:27:56 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Bruno Prémont <bonbons@...ux-vserver.org>
Cc:	Mike Frysinger <vapier.adi@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>,
	Pekka Enberg <penberg@...nel.org>
Subject: Re: 2.6.39-rc4+: Kernel leaking memory during FS scanning,
 regression?

On Tue, Apr 26, 2011 at 08:19:04AM +0200, Bruno Prémont wrote:
> On Mon, 25 Apr 2011 14:49:33 "Paul E. McKenney" wrote:
> > On Mon, Apr 25, 2011 at 02:30:02PM -0700, Linus Torvalds wrote:
> > > 2011/4/25 Bruno Prémont <bonbons@...ux-vserver.org>:
> > > >
> > > > Between 1-slabinfo and 2-slabinfo some values increased (a lot) while a few
> > > > ones did decrease. Don't know which ones are RCU-affected and which ones are
> > > > not.
> > > 
> > > It really sounds as if the tiny-rcu kthread somehow just stops
> > > handling callbacks. The ones that keep increasing do seem to be all
> > > rcu-free'd (but I didn't really check).
> > > 
> > > The thing is shown as running:
> > > 
> > > root         6  0.0  0.0      0     0 ?        R    22:14   0:00  \_
> > > [rcu_kthread]
> > > 
> > > but nothing seems to happen and the CPU time hasn't increased at all.
> > > 
> > > I dunno. Makes no  sense to me, but yeah, I'm definitely blaming
> > > tiny-rcu. Paul, any ideas?
> > 
> > So the only ways I know for something to be runnable but not run on
> > a uniprocessor are:
> > 
> > 1.	The CPU is continually busy with higher-priority work.
> > 	This doesn't make sense in this case because the system
> > 	is idle much of the time.
> > 
> > 2.	The system is hibernating.  This doesn't make sense, otherwise
> > 	"ps" wouldn't run either.
> > 
> > Any others ideas on how the heck a process can get into this state?
> > (I have thus far been completely unable to reproduce it.)
> > 
> > The process in question has a loop in rcu_kthread() in kernel/rcutiny.c.
> > This loop contains a wait_event_interruptible(), waits for a global flag
> > to become non-zero.
> > 
> > It is awakened by invoke_rcu_kthread() in that same file, which
> > simply sets the flag to 1 and does a wake_up(), all with hardirqs
> > disabled.
> > 
> > Hmmm...  One "hail mary" patch below.  What it does is make rcu_kthread
> > run at normal priority rather than at real-time priority.  This is
> > not for inclusion -- it breaks RCU priority boosting.  But well worth
> > trying.
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
> > index 0c343b9..4551824 100644
> > --- a/kernel/rcutiny.c
> > +++ b/kernel/rcutiny.c
> > @@ -314,11 +314,15 @@ EXPORT_SYMBOL_GPL(rcu_barrier_sched);
> >   */
> >  static int __init rcu_spawn_kthreads(void)
> >  {
> > +#if 0
> >  	struct sched_param sp;
> > +#endif
> >  
> >  	rcu_kthread_task = kthread_run(rcu_kthread, NULL, "rcu_kthread");
> > +#if 0
> >  	sp.sched_priority = RCU_BOOST_PRIO;
> >  	sched_setscheduler_nocheck(rcu_kthread_task, SCHED_FIFO, &sp);
> > +#endif
> >  	return 0;
> >  }
> >  early_initcall(rcu_spawn_kthreads);
> 
> I will give that patch a shot on Wednesday evening (European time) as I
> wont have enough time in front of the affected box until then to do any
> deeper testing. (same for trying to out with the other -rc kernels as
> suggested by Mike)

Thank you for both of these!!!

> Though I will use the few minutes I have this evening to try to fetch
> kernel traces of running tasks with sysrq+t which may eventually give
> us a hint at where rcu_thread is stuck/waiting.

This would be very helpful to me!

For my part, I will use some plane time today to stare at my code some
more and see what bugs I can find.

Linus, in the meantime, please feel free to revert 687d7a960 (rcu:
restrict TREE_RCU to SMP builds with !PREEMPT), which would allow anyone
not wanting to help chase this down to get on with their lives.

							Thanx, Paul
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