On Sun, Mar 17, 2013 at 9:31 PM, Shaohua Li <shli@xxxxxxxxxx> wrote:
> release_stripe still has big lock contention. We just add the stripe to a llist
> without taking device_lock. We let the raid5d thread to do the real stripe
> release, which must hold device_lock anyway. In this way, release_stripe
> doesn't hold any locks.
>
> The side effect is the released stripes order is changed. But sounds not a big
> deal, stripes are never handled in order. And I thought block layer can already
> do nice request merge, which means order isn't that important.
>
> I kept the unplug release batch, which is unnecessary with this patch from lock
> contention avoid point of view, and actually if we delete it, the stripe_head
> release_list and lru can share storage. But the unplug release batch is also
> helpful for request merge. We probably can delay wakeup raid5d till unplug, but
> I'm still afraid of the case which raid5d is running.
>
> Signed-off-by: Shaohua Li <shli@xxxxxxxxxxxx>
> ---
> drivers/md/raid5.c | 53 ++++++++++++++++++++++++++++++++++++++++++++++++-----
> drivers/md/raid5.h | 3 +++
> 2 files changed, 51 insertions(+), 5 deletions(-)
>
> Index: linux/drivers/md/raid5.c
> ===================================================================
> --- linux.orig/drivers/md/raid5.c 2013-03-18 08:49:43.276628437 +0800
> +++ linux/drivers/md/raid5.c 2013-03-18 10:33:58.561988946 +0800
> @@ -262,12 +262,45 @@ static void __release_stripe(struct r5co
> do_release_stripe(conf, sh);
> }
>
> +/* should hold conf->device_lock already */
> +static int release_stripe_list(struct r5conf *conf)
> +{
> + struct stripe_head *sh;
> + struct llist_node *node;
> + int count = 0;
> +
> + while (1) {
> + node = llist_del_first(&conf->released_stripes);
> + if (!node)
> + break;
> + sh = llist_entry(node, struct stripe_head, release_list);
Do we need a smp_mb__before_clear_bit() here to pair with the
test_and_set_bit() to preclude re-adding the stripe_head before it is
fully deleted?
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