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

[Paul E. McKenney]

On Fri, Oct 27, 2017 at 11:45:08AM +1100, NeilBrown wrote:
> On Thu, Oct 26 2017, 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.
> 
> Thanks for the confirmation 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.)
> 
> I had briefly considered that option, but it doesn't work.
> The purpose of this synchronize_rcu() is to wait for any filename lookup
> which might be locklessly touching the mountpoint to complete.
> It is only after that that the real meat of unmount happen - the
> filesystem is told that the last reference is gone, and it gets to
> flush any saved changes out to disk etc.
> That stuff really has to happen before the umount syscall returns.

Hey, I was hoping!  ;-)

						Thanx, Paul