Re: [PATCH] VFS: use synchronize_rcu_expedited() in namespace_unlock()

[Paul E. McKenney]

On Thu, Oct 26, 2017 at 05:27:43AM -0700, Paul E. McKenney wrote:
> On Thu, Oct 26, 2017 at 01:26:37PM +1100, NeilBrown wrote:
> > 
> > The synchronize_rcu() in namespace_unlock() is called every time
> > a filesystem is unmounted.  If a great many filesystems are mounted,
> > this can cause a noticable slow-down in, for example, system shutdown.
> > 
> > The sequence:
> >   mkdir -p /tmp/Mtest/{0..5000}
> >   time for i in /tmp/Mtest/*; do mount -t tmpfs tmpfs $i ; done
> >   time umount /tmp/Mtest/*
> > 
> > on a 4-cpu VM can report 8 seconds to mount the tmpfs filesystems, and
> > 100 seconds to unmount them.
> > 
> > Boot the same VM with 1 CPU and it takes 18 seconds to mount the
> > tmpfs filesystems, but only 36 to unmount.
> > 
> > If we change the synchronize_rcu() to synchronize_rcu_expedited()
> > the umount time on a 4-cpu VM is 8 seconds to mount and 0.6 to
> > unmount.
> > 
> > I think this 200-fold speed up is worth the slightly higher system
> > impact of use synchronize_rcu_expedited().
> > 
> > Signed-off-by: NeilBrown <neilb@xxxxxxxx>
> > ---
> > 
> > Cc: to Paul and Josh in case they'll correct me if using _expedited()
> > is really bad here.
> 
> I suspect that filesystem unmount is pretty rare in production real-time
> workloads, which are the ones that might care.  So I would guess that
> this is OK.
> 
> If the real-time guys ever do want to do filesystem unmounts while their
> real-time applications are running, they might modify this so that it can
> use synchronize_rcu() instead for real-time builds of the kernel.

Which they can already do using the rcupdate.rcu_normal boot parameter.

							Thanx, Paul

> But just for completeness, one way to make this work across the board
> might be to instead use call_rcu(), with the callback function kicking
> off a workqueue handler to do the rest of the unmount.  Of course,
> in saying that, I am ignoring any mutexes that you might be holding
> across this whole thing, and also ignoring any problems that might arise
> when returning to userspace with some portion of the unmount operation
> still pending.  (For example, someone unmounting a filesystem and then
> immediately remounting that same filesystem.)
> 
> 							Thanx, Paul
> 
> > Thanks,
> > NeilBrown
> > 
> > 
> >  fs/namespace.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/fs/namespace.c b/fs/namespace.c
> > index 3b601f115b6c..fce91c447fab 100644
> > --- a/fs/namespace.c
> > +++ b/fs/namespace.c
> > @@ -1420,7 +1420,7 @@ static void namespace_unlock(void)
> >  	if (likely(hlist_empty(&head)))
> >  		return;
> >  
> > -	synchronize_rcu();
> > +	synchronize_rcu_expedited();
> >  
> >  	group_pin_kill(&head);
> >  }
> > -- 
> > 2.14.0.rc0.dirty
> > 
> 
>