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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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