cfq-iosched: Always provide group isolation.

Effectively, make group_isolation=1 the default and remove the tunable.
The setting group_isolation=0 was because by default we idle on
sync-noidle tree and on fast devices, this can be very harmful for
throughput.

However, this problem can also be addressed by tuning slice_idle and
possibly group_idle on faster storage devices.

This change simplifies the CFQ code by removing the feature entirely.
Signed-off-by: Justin TerAvest <teravest@google.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

Documentation/cgroups/blkio-controller.txt

@@ -343,34 +343,6 @@ Common files among various policies
 
 CFQ sysfs tunable
 =================
-/sys/block/<disk>/queue/iosched/group_isolation
------------------------------------------------
-
-If group_isolation=1, it provides stronger isolation between groups at the
-expense of throughput. By default group_isolation is 0. In general that
-means that if group_isolation=0, expect fairness for sequential workload
-only. Set group_isolation=1 to see fairness for random IO workload also.
-
-Generally CFQ will put random seeky workload in sync-noidle category. CFQ
-will disable idling on these queues and it does a collective idling on group
-of such queues. Generally these are slow moving queues and if there is a
-sync-noidle service tree in each group, that group gets exclusive access to
-disk for certain period. That means it will bring the throughput down if
-group does not have enough IO to drive deeper queue depths and utilize disk
-capacity to the fullest in the slice allocated to it. But the flip side is
-that even a random reader should get better latencies and overall throughput
-if there are lots of sequential readers/sync-idle workload running in the
-system.
-
-If group_isolation=0, then CFQ automatically moves all the random seeky queues
-in the root group. That means there will be no service differentiation for
-that kind of workload. This leads to better throughput as we do collective
-idling on root sync-noidle tree.
-
-By default one should run with group_isolation=0. If that is not sufficient
-and one wants stronger isolation between groups, then set group_isolation=1
-but this will come at cost of reduced throughput.
-
 /sys/block/<disk>/queue/iosched/slice_idle
 ------------------------------------------
 On a faster hardware CFQ can be slow, especially with sequential workload.

block/cfq-iosched.c

@@ -146,7 +146,6 @@ struct cfq_queue {
 	struct cfq_rb_root *service_tree;
 	struct cfq_queue *new_cfqq;
 	struct cfq_group *cfqg;
-	struct cfq_group *orig_cfqg;
 	/* Number of sectors dispatched from queue in single dispatch round */
 	unsigned long nr_sectors;
 };
@@ -285,7 +284,6 @@ struct cfq_data {
 	unsigned int cfq_slice_idle;
 	unsigned int cfq_group_idle;
 	unsigned int cfq_latency;
-	unsigned int cfq_group_isolation;
 
 	unsigned int cic_index;
 	struct list_head cic_list;
@@ -1187,32 +1185,6 @@ static void cfq_service_tree_add(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 	int new_cfqq = 1;
 	int group_changed = 0;
 
-#ifdef CONFIG_CFQ_GROUP_IOSCHED
-	if (!cfqd->cfq_group_isolation
-	    && cfqq_type(cfqq) == SYNC_NOIDLE_WORKLOAD
-	    && cfqq->cfqg && cfqq->cfqg != &cfqd->root_group) {
-		/* Move this cfq to root group */
-		cfq_log_cfqq(cfqd, cfqq, "moving to root group");
-		if (!RB_EMPTY_NODE(&cfqq->rb_node))
-			cfq_group_service_tree_del(cfqd, cfqq->cfqg);
-		cfqq->orig_cfqg = cfqq->cfqg;
-		cfqq->cfqg = &cfqd->root_group;
-		cfqd->root_group.ref++;
-		group_changed = 1;
-	} else if (!cfqd->cfq_group_isolation
-		   && cfqq_type(cfqq) == SYNC_WORKLOAD && cfqq->orig_cfqg) {
-		/* cfqq is sequential now needs to go to its original group */
-		BUG_ON(cfqq->cfqg != &cfqd->root_group);
-		if (!RB_EMPTY_NODE(&cfqq->rb_node))
-			cfq_group_service_tree_del(cfqd, cfqq->cfqg);
-		cfq_put_cfqg(cfqq->cfqg);
-		cfqq->cfqg = cfqq->orig_cfqg;
-		cfqq->orig_cfqg = NULL;
-		group_changed = 1;
-		cfq_log_cfqq(cfqd, cfqq, "moved to origin group");
-	}
-#endif
-
 	service_tree = service_tree_for(cfqq->cfqg, cfqq_prio(cfqq),
 						cfqq_type(cfqq));
 	if (cfq_class_idle(cfqq)) {
@@ -2542,7 +2514,7 @@ static int cfq_dispatch_requests(struct request_queue *q, int force)
 static void cfq_put_queue(struct cfq_queue *cfqq)
 {
 	struct cfq_data *cfqd = cfqq->cfqd;
-	struct cfq_group *cfqg, *orig_cfqg;
+	struct cfq_group *cfqg;
 
 	BUG_ON(cfqq->ref <= 0);
 
@@ -2554,7 +2526,6 @@ static void cfq_put_queue(struct cfq_queue *cfqq)
 	BUG_ON(rb_first(&cfqq->sort_list));
 	BUG_ON(cfqq->allocated[READ] + cfqq->allocated[WRITE]);
 	cfqg = cfqq->cfqg;
-	orig_cfqg = cfqq->orig_cfqg;
 
 	if (unlikely(cfqd->active_queue == cfqq)) {
 		__cfq_slice_expired(cfqd, cfqq, 0);
@@ -2564,8 +2535,6 @@ static void cfq_put_queue(struct cfq_queue *cfqq)
 	BUG_ON(cfq_cfqq_on_rr(cfqq));
 	kmem_cache_free(cfq_pool, cfqq);
 	cfq_put_cfqg(cfqg);
-	if (orig_cfqg)
-		cfq_put_cfqg(orig_cfqg);
 }
 
 /*
@@ -3953,7 +3922,6 @@ static void *cfq_init_queue(struct request_queue *q)
 	cfqd->cfq_slice_idle = cfq_slice_idle;
 	cfqd->cfq_group_idle = cfq_group_idle;
 	cfqd->cfq_latency = 1;
-	cfqd->cfq_group_isolation = 0;
 	cfqd->hw_tag = -1;
 	/*
 	 * we optimistically start assuming sync ops weren't delayed in last
@@ -4029,7 +3997,6 @@ SHOW_FUNCTION(cfq_slice_sync_show, cfqd->cfq_slice[1], 1);
 SHOW_FUNCTION(cfq_slice_async_show, cfqd->cfq_slice[0], 1);
 SHOW_FUNCTION(cfq_slice_async_rq_show, cfqd->cfq_slice_async_rq, 0);
 SHOW_FUNCTION(cfq_low_latency_show, cfqd->cfq_latency, 0);
-SHOW_FUNCTION(cfq_group_isolation_show, cfqd->cfq_group_isolation, 0);
 #undef SHOW_FUNCTION
 
 #define STORE_FUNCTION(__FUNC, __PTR, MIN, MAX, __CONV)			\
@@ -4063,7 +4030,6 @@ STORE_FUNCTION(cfq_slice_async_store, &cfqd->cfq_slice[0], 1, UINT_MAX, 1);
 STORE_FUNCTION(cfq_slice_async_rq_store, &cfqd->cfq_slice_async_rq, 1,
 		UINT_MAX, 0);
 STORE_FUNCTION(cfq_low_latency_store, &cfqd->cfq_latency, 0, 1, 0);
-STORE_FUNCTION(cfq_group_isolation_store, &cfqd->cfq_group_isolation, 0, 1, 0);
 #undef STORE_FUNCTION
 
 #define CFQ_ATTR(name) \
@@ -4081,7 +4047,6 @@ static struct elv_fs_entry cfq_attrs[] = {
 	CFQ_ATTR(slice_idle),
 	CFQ_ATTR(group_idle),
 	CFQ_ATTR(low_latency),
-	CFQ_ATTR(group_isolation),
 	__ATTR_NULL
 };