Re: [PATCH] backing-dev: use synchronize_rcu_expedited instead ofsynchronize_rcu
From: Mikulas Patocka
Date: Thu Feb 02 2012 - 15:44:20 EST
On Tue, 31 Jan 2012, 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@xxxxxxxxxx>
> > ---
> > 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.
Do you have some measurable use case where the user is removing block
devices so heavily that this causes a problem?
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