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

[Al Viro]

On Fri, Oct 05, 2018 at 11:27:37AM +1000, 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 drop to 0.6 seconds
> 
> I think this 200-fold speed up is worth the slightly high system
> impact of using synchronize_rcu_expedited().
> 
> Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> (from general rcu perspective)
> Signed-off-by: NeilBrown <neilb@xxxxxxxx>
> ---
> 
> I posted this last October, then again last November (cc:ing Linus)
> Paul is happy enough with it, but no other response.
> I'm hoping it can get applied this time....

Umm...  IIRC, the last one got sidetracked on the other thing in the series...
<checks> that was s_anon stuff.  I can live with this one; FWIW, what kind
of load would trigger the impact of the change?  Paul?