On Fri, Oct 05 2018, 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 <paul...@linux.vnet.ibm.com> (from general rcu > perspective) > Signed-off-by: NeilBrown <ne...@suse.com> > --- > > 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....
Hi Al, this isn't in 4.20-rc1. Are you still waiting for something? Thanks, NeilBrown > > Thanks, > NeilBrown > > > fs/namespace.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/fs/namespace.c b/fs/namespace.c > index 99186556f8d3..02e978b22294 100644 > --- a/fs/namespace.c > +++ b/fs/namespace.c > @@ -1360,7 +1360,7 @@ 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
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