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

[Corrado Zoccolo <czoccolo@gmail.com>]

Disabled idle window cause 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),
unconditionally 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 ebab60c..576e92d 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1981,10 +1981,14 @@ 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->cfq_latency && 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 = cfqd->cfq_slice_idle;
+               if (CIC_SEEKY(cic))
+                       idle_time = CFQ_MIN_TT;
+
+               if (cic->ttime_mean > idle_time)
                        enable_idle = 0;
                else
                        enable_idle = 1;