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


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