linux-kernel - Re: [PATCH 2/2]block cfq: don't use atomic_t for cfq

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Date:	Fri, 24 Dec 2010 08:40:59 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Jens Axboe <jaxboe@...ionio.com>,
	"jmoyer@...hat.com" <jmoyer@...hat.com>
Subject: Re: [PATCH 2/2]block cfq: don't use atomic_t for cfq_group

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@...el.com>
Reviewed-by: Jeff Moyer <jmoyer@...hat.com>
---
 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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