linux-kernel - Re: [PATCH] backing-dev: use synchronize_rcu_expedited instead of synchronize

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Date:	Tue, 31 Jan 2012 21:34:23 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	axboe@...nel.dk, "Alasdair G. Kergon" <agk@...hat.com>,
	dm-devel@...hat.com, linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ibm.com>
Subject: Re: [PATCH] backing-dev: use synchronize_rcu_expedited instead of
 synchronize_rcu

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@...hat.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.
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