[RFC PATCH 2/2] block/genhd: add sysfs knobs for the CPU latency PM QoS settings

[Tero Kristo <tero.kristo@xxxxxxxxxxxxxxx>]

Add sysfs knobs for the following parameters:

  cpu_lat_limit_us: for limiting the CPU latency to given value when block IO
		    is running
  cpu_lat_timeout_ms: for clearing up the CPU latency limit after block IO
		      is complete

This can be used to prevent the CPU from entering deep idle states when
block IO is running and waiting for an interrupt, potentially causing
large latencies to the operation.

Signed-off-by: Tero Kristo <tero.kristo@xxxxxxxxxxxxxxx>
---
 block/genhd.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 46 insertions(+)

diff --git a/block/genhd.c b/block/genhd.c
index 8f1f3c6b4d67..2fbd967a3e36 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -1046,6 +1046,48 @@ static ssize_t partscan_show(struct device *dev,
 	return sprintf(buf, "%u\n", disk_has_partscan(dev_to_disk(dev)));
 }
 
+static ssize_t cpu_lat_limit_us_show(struct device *dev,
+				     struct device_attribute *attr, char *buf)
+{
+	struct block_device *bdev = dev_to_bdev(dev);
+
+	return sprintf(buf, "%d\n", bdev->cpu_lat_limit);
+}
+
+static ssize_t cpu_lat_limit_us_store(struct device *dev,
+				      struct device_attribute *attr,
+				      const char *buf, size_t count)
+{
+	struct block_device *bdev = dev_to_bdev(dev);
+	int i;
+
+	if (count > 0 && !kstrtoint(buf, 10, &i))
+		bdev->cpu_lat_limit = i;
+
+	return count;
+}
+
+static ssize_t cpu_lat_timeout_ms_show(struct device *dev,
+				       struct device_attribute *attr, char *buf)
+{
+	struct block_device *bdev = dev_to_bdev(dev);
+
+	return sprintf(buf, "%d\n", bdev->cpu_lat_timeout);
+}
+
+static ssize_t cpu_lat_timeout_ms_store(struct device *dev,
+					struct device_attribute *attr,
+					const char *buf, size_t count)
+{
+	struct block_device *bdev = dev_to_bdev(dev);
+	int i;
+
+	if (count > 0 && !kstrtoint(buf, 10, &i))
+		bdev->cpu_lat_timeout = i;
+
+	return count;
+}
+
 static DEVICE_ATTR(range, 0444, disk_range_show, NULL);
 static DEVICE_ATTR(ext_range, 0444, disk_ext_range_show, NULL);
 static DEVICE_ATTR(removable, 0444, disk_removable_show, NULL);
@@ -1060,6 +1102,8 @@ static DEVICE_ATTR(inflight, 0444, part_inflight_show, NULL);
 static DEVICE_ATTR(badblocks, 0644, disk_badblocks_show, disk_badblocks_store);
 static DEVICE_ATTR(diskseq, 0444, diskseq_show, NULL);
 static DEVICE_ATTR(partscan, 0444, partscan_show, NULL);
+static DEVICE_ATTR_RW(cpu_lat_limit_us);
+static DEVICE_ATTR_RW(cpu_lat_timeout_ms);
 
 #ifdef CONFIG_FAIL_MAKE_REQUEST
 ssize_t part_fail_show(struct device *dev,
@@ -1111,6 +1155,8 @@ static struct attribute *disk_attrs[] = {
 	&dev_attr_events_poll_msecs.attr,
 	&dev_attr_diskseq.attr,
 	&dev_attr_partscan.attr,
+	&dev_attr_cpu_lat_limit_us.attr,
+	&dev_attr_cpu_lat_timeout_ms.attr,
 #ifdef CONFIG_FAIL_MAKE_REQUEST
 	&dev_attr_fail.attr,
 #endif
-- 
2.43.1