[PATCH] cfq: enable idle for seeky processes on rotational NCQ devices

[Corrado Zoccolo <czoccolo@gmail.com>]

Idle window is currently disabled for seeky processes on all NCQ devices.
This is causing large latencies when seeky processes are competing with async writes,
for rotational NCQ devices.

This patch, based on Vivek Goyal's original idea (http://lkml.org/lkml/2009/7/12/110),
enables idle window for seeky processes on rotational NCQ devices.
As for non-NCQ devices, a smaller idle window (2ms) is used
for seeky processes compared to normal I/O (8ms).

RAIDs should be marked as non-rotational as well (and probably a better flag
name should be devised), since they can carry multiple operations in parallel.

Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
---
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 1ca813b..7116f11 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1951,10 +1951,12 @@ cfq_update_idle_window(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 	enable_idle = old_idle = cfq_cfqq_idle_window(cfqq);
 
 	if (!atomic_read(&cic->ioc->nr_tasks) || !cfqd->cfq_slice_idle ||
-	    (cfqd->hw_tag && CIC_SEEKY(cic)))
+	    (blk_queue_nonrot(cfqd->queue) && cfqd->hw_tag && CIC_SEEKY(cic)))
 		enable_idle = 0;
 	else if (sample_valid(cic->ttime_samples)) {
-		if (cic->ttime_mean > cfqd->cfq_slice_idle)
+		unsigned idle_time = CIC_SEEKY(cic) ? CFQ_MIN_TT
+			: cfqd->cfq_slice_idle;
+		if (cic->ttime_mean > idle_time)
 			enable_idle = 0;
 		else
 			enable_idle = 1;