Re: rcu: endless stalls

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

On Mon, Jun 11, 2012 at 07:20:14PM +0200, Mike Galbraith wrote:
> On Mon, 2012-06-11 at 16:22 +0200, Mike Galbraith wrote: 
> > On Mon, 2012-06-11 at 06:39 -0700, Paul E. McKenney wrote: 
> > > On Mon, Jun 11, 2012 at 12:06:16PM +0200, Mike Galbraith wrote:
> > > > Greetings,
> > > > 
> > > > I received a report of a 48 core UV box hitting a gripe, taking longer
> > > > than timeout to emit same, so box griped endlessly, forcing reboot.
> > > 
> > > So it took minutes to print an RCU CPU stall warning?  On only 48 CPUs?
> > > 
> > > If so, yow!!!
> > 
> > The report was a tad fuzzy on details, but serial console can take a
> > while at low baud rate, and the box was apparently perma-stalled.
> 
> Gee, no wonder it took a while to emit the gripe, there were a few more
> than 48 CPUs actually.. 4048 more to be precise.
> 
> 4096 tasks on 4096 CPUs all hammering wait_lock at once r!pretty.

That would be bad...

BTW, one other thing on the patch -- it needs to use a separate flag,
otherwise there are races with sysfs and panic() updates to the flag.

> 2aa15890 - mm: prevent concurrent unmap_mapping_range() on the same inode

I confess, you lost me on this one.  You believe that this commit is
the cause of the RCU CPU stall warnings?

							Thanx, Paul