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 <[email protected]> (from general rcu
> perspective)
> Signed-off-by: NeilBrown <[email protected]>
> ---
>
> 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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