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Subject[PATCH 2/3] cfq-iosched: Fair cross-group preemption (implementation)
Date
Add a new function cfq_group_should_preempt() which uses the new
service classes to decide if a task in one cgroup should preempt a
task in another. We only allow it if the preempting task's vdisktime
is "behind" the preempted task.

Signed-off-by: Chad Talbott <ctalbott@google.com>
---
block/cfq-iosched.c | 41 ++++++++++++++++++++++++++++++++++++++---
1 files changed, 38 insertions(+), 3 deletions(-)
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 55e78b7..dfcce80 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1157,6 +1157,32 @@ void cfq_unlink_blkio_group(void *key, struct blkio_group *blkg)
spin_unlock_irqrestore(cfqd->queue->queue_lock, flags);
}

+/*
+ * Return true if new_cfqg should preempt cfqg. A return value of
+ * false means "don't care." In that case, cfq has other heuristics
+ * to decide
+ */
+static bool
+cfq_group_should_preempt(struct cfq_queue *new_cfqq, struct cfq_queue *cfqq,
+ struct request *rq)
+{
+ struct cfq_group *cfqg = cfqq->cfqg;
+ struct cfq_group *new_cfqg = new_cfqq->cfqg;
+ u64 grace_period;
+
+ /* in-group preemption is handled elsewhere */
+ if (new_cfqg == cfqg)
+ return false;
+
+ if (!(new_cfqg->class == BLKIO_RT_CLASS &&
+ cfqg->class == BLKIO_BE_CLASS))
+ return false;
+
+ grace_period = cfq_scale_slice(cfqq->allocated_slice, cfqg);
+ return time_before64(new_cfqg->vdisktime,
+ cfqg->vdisktime + grace_period);
+}
+
#else /* GROUP_IOSCHED */
static struct cfq_group *cfq_get_cfqg(struct cfq_data *cfqd, int create)
{
@@ -1176,6 +1202,12 @@ cfq_link_cfqq_cfqg(struct cfq_queue *cfqq, struct cfq_group *cfqg) {
static void cfq_release_cfq_groups(struct cfq_data *cfqd) {}
static inline void cfq_put_cfqg(struct cfq_group *cfqg) {}

+static bool
+cfq_group_should_preempt(struct cfq_queue *new_cfqq, struct cfq_queue *cfqq,
+ struct request *rq)
+{
+ return false;
+}
#endif /* GROUP_IOSCHED */

/*
@@ -3234,6 +3266,12 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
if (!cfqq)
return false;

+ if (cfq_group_should_preempt(new_cfqq, cfqq, rq))
+ return true;
+
+ if (new_cfqq->cfqg != cfqq->cfqg)
+ return false;
+
if (cfq_class_idle(new_cfqq))
return false;

@@ -3253,9 +3291,6 @@ 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;

--
1.7.3.1


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