Re: [PATCH 2/2]block cfq: don't use atomic_t for cfq_group

From: Shaohua Li
Date: Thu Dec 23 2010 - 19:41:08 EST


On Thu, 2010-12-23 at 23:18 +0800, Vivek Goyal wrote:
> On Thu, Dec 23, 2010 at 10:45:35AM +0800, Shaohua Li wrote:
> > cfq_group->ref is used with queue_lock hold, the only exception is
> > cfq_set_request, which looks like a bug to me, so ref doesn't need
> > to be an atomic and atomic operation is slower.
> >
>
> [..]
> >
> > @@ -3683,12 +3685,13 @@ new_queue:
> >
> > cfqq->allocated[rw]++;
> > cfqq->ref++;
> > + cfqg = cfq_ref_get_cfqg(cfqq->cfqg);
> >
> > spin_unlock_irqrestore(q->queue_lock, flags);
> >
> > rq->elevator_private = cic;
> > rq->elevator_private2 = cfqq;
> > - rq->elevator_private3 = cfq_ref_get_cfqg(cfqq->cfqg);
> > + rq->elevator_private3 = cfqg;
>
> I think you can move every thing under spinlock. IOW, first set the
> rq->elevator_private* fields and delay the release of spinlock. Few
> days back I was also looking at wondering that why are we releasing
> the spinlock early.
sure, it's harmless anyway and more readable. Updated the patch.

cfq_group->ref is used with queue_lock hold, the only exception is
cfq_set_request, which looks like a bug to me, so ref doesn't need
to be an atomic and atomic operation is slower.

Signed-off-by: Shaohua Li <shaohua.li@xxxxxxxxx>
Reviewed-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
---
block/cfq-iosched.c | 23 ++++++++++++-----------
1 file changed, 12 insertions(+), 11 deletions(-)

Index: linux/block/cfq-iosched.c
===================================================================
--- linux.orig/block/cfq-iosched.c 2010-12-24 08:32:19.000000000 +0800
+++ linux/block/cfq-iosched.c 2010-12-24 08:33:10.000000000 +0800
@@ -209,7 +209,7 @@ struct cfq_group {
struct blkio_group blkg;
#ifdef CONFIG_CFQ_GROUP_IOSCHED
struct hlist_node cfqd_node;
- atomic_t ref;
+ int ref;
#endif
/* number of requests that are on the dispatch list or inside driver */
int dispatched;
@@ -1026,7 +1026,7 @@ cfq_find_alloc_cfqg(struct cfq_data *cfq
* elevator which will be dropped by either elevator exit
* or cgroup deletion path depending on who is exiting first.
*/
- atomic_set(&cfqg->ref, 1);
+ cfqg->ref = 1;

/*
* Add group onto cgroup list. It might happen that bdi->dev is
@@ -1071,7 +1071,7 @@ static struct cfq_group *cfq_get_cfqg(st

static inline struct cfq_group *cfq_ref_get_cfqg(struct cfq_group *cfqg)
{
- atomic_inc(&cfqg->ref);
+ cfqg->ref++;
return cfqg;
}

@@ -1083,7 +1083,7 @@ static void cfq_link_cfqq_cfqg(struct cf

cfqq->cfqg = cfqg;
/* cfqq reference on cfqg */
- atomic_inc(&cfqq->cfqg->ref);
+ cfqq->cfqg->ref++;
}

static void cfq_put_cfqg(struct cfq_group *cfqg)
@@ -1091,8 +1091,9 @@ static void cfq_put_cfqg(struct cfq_grou
struct cfq_rb_root *st;
int i, j;

- BUG_ON(atomic_read(&cfqg->ref) <= 0);
- if (!atomic_dec_and_test(&cfqg->ref))
+ BUG_ON(cfqg->ref <= 0);
+ cfqg->ref--;
+ if (cfqg->ref)
return;
for_each_cfqg_st(cfqg, i, j, st)
BUG_ON(!RB_EMPTY_ROOT(&st->rb) || st->active != NULL);
@@ -1200,7 +1201,7 @@ static void cfq_service_tree_add(struct
cfq_group_service_tree_del(cfqd, cfqq->cfqg);
cfqq->orig_cfqg = cfqq->cfqg;
cfqq->cfqg = &cfqd->root_group;
- atomic_inc(&cfqd->root_group.ref);
+ cfqd->root_group.ref++;
group_changed = 1;
} else if (!cfqd->cfq_group_isolation
&& cfqq_type(cfqq) == SYNC_WORKLOAD && cfqq->orig_cfqg) {
@@ -3683,12 +3684,12 @@ new_queue:

cfqq->allocated[rw]++;
cfqq->ref++;
-
- spin_unlock_irqrestore(q->queue_lock, flags);
-
rq->elevator_private = cic;
rq->elevator_private2 = cfqq;
rq->elevator_private3 = cfq_ref_get_cfqg(cfqq->cfqg);
+
+ spin_unlock_irqrestore(q->queue_lock, flags);
+
return 0;

queue_fail:
@@ -3886,7 +3887,7 @@ static void *cfq_init_queue(struct reque
* Take a reference to root group which we never drop. This is just
* to make sure that cfq_put_cfqg() does not try to kfree root group
*/
- atomic_set(&cfqg->ref, 1);
+ cfqg->ref = 1;
rcu_read_lock();
cfq_blkiocg_add_blkio_group(&blkio_root_cgroup, &cfqg->blkg,
(void *)cfqd, 0);


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