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SubjectRe: rcu: endless stalls
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On Mon, 2012-06-11 at 11:01 -0700, Paul E. McKenney wrote: 
> 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?

4096 tasks on 4096 CPUs exit (well, try to) simultaneously.

Call Trace:
__mutex_lock_slowpath+0x94/0x150
mutex_lock+0x1a/0x40
unlink_file_vma+0x3f/0xf0
free_pgtables+0x40/0x100
exit_mmap+0xb0/0x120
mmput+0x49/0x120
exit_mm+0x122/0x160
do_exit+0x179/0x8d0
do_group_exit+0x3d/0xb0
sys_exit_group+0x12/0x20

Monster box dies screaming.

-Mike



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