Re: [PATCH] backing-dev: use synchronize_rcu_expedited instead of synchronize_rcu

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Jan 31, 2012 at 09:34:23PM +0100, Peter Zijlstra wrote:
> On Wed, 2011-07-20 at 20:29 -0400, Mikulas Patocka wrote:
> > Hi Jens
> > 
> > Please would you consider taking this into the block tree? It seems to 
> > speed up device deletion enormously.
> > 
> > Mikulas
> > 
> > ---
> > 
> > backing-dev: use synchronize_rcu_expedited instead of synchronize_rcu
> > 
> > synchronize_rcu sleeps several timer ticks. synchronize_rcu_expedited is 
> > much faster.
> > 
> > With 100Hz timer frequency, when we remove 10000 block devices with 
> > "dmsetup remove_all" command, it takes 27 minutes. With this patch, 
> > removing 10000 block devices takes only 15 seconds.
> > 
> > Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> > 
> > ---
> >  mm/backing-dev.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > Index: linux-3.0-rc7-fast/mm/backing-dev.c
> > ===================================================================
> > --- linux-3.0-rc7-fast.orig/mm/backing-dev.c	2011-07-19 18:01:00.000000000 +0200
> > +++ linux-3.0-rc7-fast/mm/backing-dev.c	2011-07-19 18:01:07.000000000 +0200
> > @@ -505,7 +505,7 @@ static void bdi_remove_from_list(struct 
> >  	list_del_rcu(&bdi->bdi_list);
> >  	spin_unlock_bh(&bdi_lock);
> >  
> > -	synchronize_rcu();
> > +	synchronize_rcu_expedited();
> >  }
> >  
> 
> Urgh, I just noticed this crap in my tree.. You realize that what you're
> effectively hammering a global sync primitive this way? Depending on
> what RCU flavour you have any SMP variant will at least do a machine
> wide IPI broadcast for every sync_rcu_exp(), some do significantly more.
> 
> The much better solution would've been to batch your block-dev removals
> and use a single sync_rcu as barrier.
> 
> This is not cool.

Indeed, synchronize_rcu_expedited() is quite heavyweight, so as Peter
suggests, if you can use batching you will get even better performance
with much less load on the rest of the system.

							Thanx, Paul