block: add block polling support

Add basic support for polling for specific IO to complete. This uses
the cookie that blk-mq passes back, which enables the block layer
to pass this cookie to the driver to spin for a specific request.

This will be combined with request latency tracking, so we can make
qualified decisions about when to poll and when not to. For now, for
benchmark purposes, we add a sysfs file that controls whether polling
is enabled or not.

Signed-off-by: Jens Axboe <axboe@fb.com>
Acked-by: Christoph Hellwig <hch@lst.de>
Acked-by: Keith Busch <keith.busch@intel.com>

1 files changed, 35 insertions, 0 deletions

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 31849e328b45..565b8dac5782 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -317,6 +317,34 @@ queue_rq_affinity_store(struct request_queue *q, const char *page, size_t count)
         return ret;
 }
 
+static ssize_t queue_poll_show(struct request_queue *q, char *page)
+{
+        return queue_var_show(test_bit(QUEUE_FLAG_POLL, &q->queue_flags), page);
+}
+
+static ssize_t queue_poll_store(struct request_queue *q, const char *page,
+                                size_t count)
+{
+        unsigned long poll_on;
+        ssize_t ret;
+
+        if (!q->mq_ops || !q->mq_ops->poll)
+                return -EINVAL;
+
+        ret = queue_var_store(&poll_on, page, count);
+        if (ret < 0)
+                return ret;
+
+        spin_lock_irq(q->queue_lock);
+        if (poll_on)
+                queue_flag_set(QUEUE_FLAG_POLL, q);
+        else
+                queue_flag_clear(QUEUE_FLAG_POLL, q);
+        spin_unlock_irq(q->queue_lock);
+
+        return ret;
+}
+
 static struct queue_sysfs_entry queue_requests_entry = {
         .attr = {.name = "nr_requests", .mode = S_IRUGO | S_IWUSR },
         .show = queue_requests_show,
@@ -442,6 +470,12 @@ static struct queue_sysfs_entry queue_random_entry = {
         .store = queue_store_random,
 };
 
+static struct queue_sysfs_entry queue_poll_entry = {
+        .attr = {.name = "io_poll", .mode = S_IRUGO | S_IWUSR },
+        .show = queue_poll_show,
+        .store = queue_poll_store,
+};
+
 static struct attribute *default_attrs[] = {
         &queue_requests_entry.attr,
         &queue_ra_entry.attr,
@@ -466,6 +500,7 @@ static struct attribute *default_attrs[] = {
         &queue_rq_affinity_entry.attr,
         &queue_iostats_entry.attr,
         &queue_random_entry.attr,
+        &queue_poll_entry.attr,
         NULL,
 };