Patch "cfq-iosched: fix the delay of cfq_group's vdisktime under iops mode" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

cfq-iosched: fix the delay of cfq_group's vdisktime under iops mode

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
cfq-iosched-fix-the-delay-of-cfq_group-s-vdisktime-under-iops-mode.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Hou Tao <[email protected]>
Date: Wed, 1 Mar 2017 09:02:33 +0800
Subject: cfq-iosched: fix the delay of cfq_group's vdisktime under iops mode

From: Hou Tao <[email protected]>

commit 5be6b75610cefd1e21b98a218211922c2feb6e08 upstream.

When adding a cfq_group into the cfq service tree, we use CFQ_IDLE_DELAY as the delay of cfq_group's vdisktime if there have been other cfq_groups already.

When cfq is under iops mode, commit 9a7f38c42c2b ("cfq-iosched: Convert from jiffies to nanoseconds") could result in a large iops delay and lead to an abnormal io schedule delay for the added cfq_group. To fix it, we just need to revert to the old CFQ_IDLE_DELAY value: HZ / 5 when iops mode is enabled.

Despite having the same value, the delay of a cfq_queue in idle class and the delay of cfq_group are different things, so I define two new macros for the delay of a cfq_group under time-slice mode and iops mode.

---
 block/cfq-iosched.c |   17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

@@ -36,9 +36,13 @@ static const u64 cfq_target_latency = (u
 static const int cfq_hist_divisor = 4;
 
 /*
- * offset from end of service tree
+ * offset from end of queue service tree for idle class
  */
 #define CFQ_IDLE_DELAY		(NSEC_PER_SEC / 5)
+/* offset from end of group service tree under time slice mode */
+#define CFQ_SLICE_MODE_GROUP_DELAY (NSEC_PER_SEC / 5)
+/* offset from end of group service under IOPS mode */
+#define CFQ_IOPS_MODE_GROUP_DELAY (HZ / 5)
 
 /*
  * below this threshold, we consider thinktime immediate

@@ -1370,6 +1374,14 @@ cfq_group_service_tree_add(struct cfq_rb
 	cfqg->vfraction = max_t(unsigned, vfr, 1);
 }
 
+static inline u64 cfq_get_cfqg_vdisktime_delay(struct cfq_data *cfqd)
+{
+	if (!iops_mode(cfqd))
+		return CFQ_SLICE_MODE_GROUP_DELAY;
+	else
+		return CFQ_IOPS_MODE_GROUP_DELAY;
+}
+
 static void
 cfq_group_notify_queue_add(struct cfq_data *cfqd, struct cfq_group *cfqg)
 {

@@ -1389,7 +1401,8 @@ cfq_group_notify_queue_add(struct cfq_da
 	n = rb_last(&st->rb);
 	if (n) {
 		__cfqg = rb_entry_cfqg(n);
-		cfqg->vdisktime = __cfqg->vdisktime + CFQ_IDLE_DELAY;
+		cfqg->vdisktime = __cfqg->vdisktime +
+			cfq_get_cfqg_vdisktime_delay(cfqd);
 	} else
 		cfqg->vdisktime = st->min_vdisktime;
 	cfq_group_service_tree_add(st, cfqg);

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/cfq-iosched-fix-the-delay-of-cfq_group-s-vdisktime-under-iops-mode.patch