linux-kernel - Re: [PATCH - resend] VFS: use synchronize_rcu_expedited() in namespace

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Date:   Tue, 06 Nov 2018 14:15:35 +1100
From:   NeilBrown <neilb@...e.com>
To:     Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     paulmck@...ux.vnet.ibm.com, Florian Weimer <fweimer@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>
Subject: Re: [PATCH - resend] VFS: use synchronize_rcu_expedited() in namespace_unlock()

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 <paulmck@...ux.vnet.ibm.com> (from general rcu perspective)
> Signed-off-by: NeilBrown <neilb@...e.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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