[PATCH] cfq-iosched: allow groups preemption for sync-noidle workloads

[Konstantin Khlebnikov <khlebnikov@openvz.org>]

commit v2.6.32-102-g8682e1f "blkio: Provide some isolation between groups" break
fast switching between task and journal-thread for very common write-fsync workload.
cfq wait idle slice at each cfqq switch, if this task is from non-root blkio cgroup.

This patch move idling sync-noidle preempting check little bit upwards and update
new service_tree->count check for case with two different groups.
I do not quite understand what means these check for new_cfqq, but now it even works.

Without patch I got 49 iops and with this patch 798, for this trivial fio script:

[write-fsync]
cgroup=test
cgroup_weight=1000
rw=write
fsync=1
size=100m
runtime=10s

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
---
 block/cfq-iosched.c |   14 +++++++-------
 1 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 3c7b537..c71533e 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -3318,19 +3318,19 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
 	if (rq_is_sync(rq) && !cfq_cfqq_sync(cfqq))
 		return true;
 
-	if (new_cfqq->cfqg != cfqq->cfqg)
-		return false;
-
-	if (cfq_slice_used(cfqq))
-		return true;
-
 	/* Allow preemption only if we are idling on sync-noidle tree */
 	if (cfqd->serving_type == SYNC_NOIDLE_WORKLOAD &&
 	    cfqq_type(new_cfqq) == SYNC_NOIDLE_WORKLOAD &&
-	    new_cfqq->service_tree->count == 2 &&
+	    new_cfqq->service_tree->count == 1+(new_cfqq->cfqg == cfqq->cfqg) &&
 	    RB_EMPTY_ROOT(&cfqq->sort_list))
 		return true;
 
+	if (new_cfqq->cfqg != cfqq->cfqg)
+		return false;
+
+	if (cfq_slice_used(cfqq))
+		return true;
+
 	/*
 	 * So both queues are sync. Let the new request get disk time if
 	 * it's a metadata request and the current queue is doing regular IO.