[PATCH] io-controller: optimization for iog deletion when elevator exiting

[Gui Jianfeng <guijianfeng@cn.fujitsu.com>]

Hi Vivek,

There's no need to travel the iocg->group_data for each iog
when exiting a elevator, that costs too much. An alternative 
solution is reseting iocg_id as soon as an io group unlinking 
from a iocg. Make a decision that whether it's need to carry 
out deleting action by checking iocg_id.

Signed-off-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
---
 block/elevator-fq.c |   29 ++++++++++-------------------
 1 files changed, 10 insertions(+), 19 deletions(-)

diff --git a/block/elevator-fq.c b/block/elevator-fq.c
index d779282..b26fe0f 100644
--- a/block/elevator-fq.c
+++ b/block/elevator-fq.c
@@ -2218,8 +2218,6 @@ void io_group_cleanup(struct io_group *iog)
 	BUG_ON(iog->sched_data.active_entity != NULL);
 	BUG_ON(entity != NULL && entity->tree != NULL);
 
-	iog->iocg_id = 0;
-
 	/*
 	 * Wait for any rcu readers to exit before freeing up the group.
 	 * Primarily useful when io_get_io_group() is called without queue
@@ -2376,6 +2374,7 @@ remove_entry:
 			  group_node);
 	efqd = rcu_dereference(iog->key);
 	hlist_del_rcu(&iog->group_node);
+	iog->iocg_id = 0;
 	spin_unlock_irqrestore(&iocg->lock, flags);
 
 	spin_lock_irqsave(efqd->queue->queue_lock, flags);
@@ -2403,35 +2402,27 @@ done:
 void io_group_check_and_destroy(struct elv_fq_data *efqd, struct io_group *iog)
 {
 	struct io_cgroup *iocg;
-	unsigned short id = iog->iocg_id;
-	struct hlist_node *n;
-	struct io_group *__iog;
 	unsigned long flags;
 	struct cgroup_subsys_state *css;
 
 	rcu_read_lock();
 
-	BUG_ON(!id);
-	css = css_lookup(&io_subsys, id);
+	css = css_lookup(&io_subsys, iog->iocg_id);
 
-	/* css can't go away as associated io group is still around */
-	BUG_ON(!css);
+	if (!css)
+		goto out;
 
 	iocg = container_of(css, struct io_cgroup, css);
 
 	spin_lock_irqsave(&iocg->lock, flags);
-	hlist_for_each_entry_rcu(__iog, n, &iocg->group_data, group_node) {
-		/*
-		 * Remove iog only if it is still in iocg list. Cgroup
-		 * deletion could have deleted it already.
-		 */
-		if (__iog == iog) {
-			hlist_del_rcu(&iog->group_node);
-			__io_destroy_group(efqd, iog);
-			break;
-		}
+
+	if (iog->iocg_id) {
+		hlist_del_rcu(&iog->group_node);
+		__io_destroy_group(efqd, iog);
 	}
+
 	spin_unlock_irqrestore(&iocg->lock, flags);
+out:
 	rcu_read_unlock();
 }
 
-- 1.5.4.rc3