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Subject[PATCH 3/6] blk-cgroup: buffered write IO controller - bandwidth weight
blkcg->weight can be trivially supported for buffered writes by directly
scaling task_ratelimit in balance_dirty_pages().

However the sementics are not quite the same with direct IO.

- for direct IO, weight is normally applied to disk time
- for buffered writes, weight is applied to dirty rate

Notes about the (balanced_dirty_ratelimit > write_bw) check removal:
When there is only one dd running and its weight is set to
BLKIO_WEIGHT_MIN=10, bdi->dirty_ratelimit will end up balancing around

write_bw * BLKIO_WEIGHT_DEFAULT / BLKIO_WEIGHT_MIN
= write_bw * 50

So the limit should now be raised to (write_bw * 50) to deal with the
above extreme case, which seems too large to be useful for normal cases.
So just remove it.

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
---
include/linux/blk-cgroup.h | 18 ++++++++++++++----
mm/page-writeback.c | 10 +++++-----
2 files changed, 19 insertions(+), 9 deletions(-)
--- linux-next.orig/include/linux/blk-cgroup.h 2012-03-28 13:42:26.686233288 +0800
+++ linux-next/include/linux/blk-cgroup.h 2012-03-28 14:25:16.150180560 +0800
@@ -21,6 +21,10 @@ enum blkio_policy_id {
BLKIO_POLICY_THROTL, /* Throttling */
};

+#define BLKIO_WEIGHT_MIN 10
+#define BLKIO_WEIGHT_MAX 1000
+#define BLKIO_WEIGHT_DEFAULT 500
+
/* Max limits for throttle policy */
#define THROTL_IOPS_MAX UINT_MAX

@@ -209,6 +213,11 @@ extern unsigned int blkcg_get_read_iops(
extern unsigned int blkcg_get_write_iops(struct blkio_cgroup *blkcg,
dev_t dev);

+static inline unsigned int blkcg_weight(struct blkio_cgroup *blkcg)
+{
+ return blkcg->weight;
+}
+
typedef void (blkio_unlink_group_fn) (void *key, struct blkio_group *blkg);

typedef void (blkio_update_group_weight_fn) (void *key,
@@ -259,11 +268,12 @@ static inline void blkio_policy_unregist

static inline char *blkg_path(struct blkio_group *blkg) { return NULL; }

-#endif
+static inline unsigned int blkcg_weight(struct blkio_cgroup *blkcg)
+{
+ return BLKIO_WEIGHT_DEFAULT;
+}

-#define BLKIO_WEIGHT_MIN 10
-#define BLKIO_WEIGHT_MAX 1000
-#define BLKIO_WEIGHT_DEFAULT 500
+#endif

#ifdef CONFIG_DEBUG_BLK_CGROUP
void blkiocg_update_avg_queue_size_stats(struct blkio_group *blkg);
--- linux-next.orig/mm/page-writeback.c 2012-03-28 13:42:26.678233289 +0800
+++ linux-next/mm/page-writeback.c 2012-03-28 14:26:02.694179605 +0800
@@ -905,11 +905,6 @@ static void bdi_update_dirty_ratelimit(s
*/
balanced_dirty_ratelimit = div_u64((u64)task_ratelimit * write_bw,
dirty_rate | 1);
- /*
- * balanced_dirty_ratelimit ~= (write_bw / N) <= write_bw
- */
- if (unlikely(balanced_dirty_ratelimit > write_bw))
- balanced_dirty_ratelimit = write_bw;

/*
* We could safely do this and return immediately:
@@ -1263,6 +1258,11 @@ static void balance_dirty_pages(struct a
bdi_thresh, bdi_dirty);
task_ratelimit = ((u64)dirty_ratelimit * pos_ratio) >>
RATELIMIT_CALC_SHIFT;
+
+ if (blkcg_weight(blkcg) != BLKIO_WEIGHT_DEFAULT)
+ task_ratelimit = (u64)task_ratelimit *
+ blkcg_weight(blkcg) / BLKIO_WEIGHT_DEFAULT;
+
max_pause = bdi_max_pause(bdi, bdi_dirty);
min_pause = bdi_min_pause(bdi, max_pause,
task_ratelimit, dirty_ratelimit,



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