[PATCH 3/3] block: introducing a bias over deadline's fifo_time

[zhaoyang.huang]

From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>

According to current policy, RT tasks possess the privilege for both of
CPU and IO scheduler which could have the preempted CFS tasks suffer big
IO-latency unfairly. This commit introduce an approximate method to
deduct the preempt affection.

TaskA
sched in
|
|
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submit_bio
|
|
|
fifo_time = jiffies + expire
(insert_request)

TaskB
sched in
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preempted by RT task
|\
| This period time is unfair to TaskB's IO request, should be adjust
|/
submit_bio
|
|
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fifo_time = jiffies + expire * CFS_PROPORTION(rq)
(insert_request)

Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
---
 block/mq-deadline.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/block/mq-deadline.c b/block/mq-deadline.c
index f958e79277b8..43c08c3d6f18 100644
--- a/block/mq-deadline.c
+++ b/block/mq-deadline.c
@@ -15,6 +15,7 @@
 #include <linux/compiler.h>
 #include <linux/rbtree.h>
 #include <linux/sbitmap.h>
+#include "../kernel/sched/sched.h"
 
 #include <trace/events/block.h>
 
@@ -802,6 +803,7 @@ static void dd_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
 	u8 ioprio_class = IOPRIO_PRIO_CLASS(ioprio);
 	struct dd_per_prio *per_prio;
 	enum dd_prio prio;
+	int fifo_expire;
 
 	lockdep_assert_held(&dd->lock);
 
@@ -840,7 +842,9 @@ static void dd_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
 		/*
 		 * set expire time and add to fifo list
 		 */
-		rq->fifo_time = jiffies + dd->fifo_expire[data_dir];
+		fifo_expire = task_is_realtime(current) ? dd->fifo_expire[data_dir] :
+			CFS_PROPORTION(current, dd->fifo_expire[data_dir]);
+		rq->fifo_time = jiffies + fifo_expire;
 		insert_before = &per_prio->fifo_list[data_dir];
 #ifdef CONFIG_BLK_DEV_ZONED
 		/*
-- 
2.25.1