block: add rq_qos_wait to rq_qos

Originally when I split out the common code from blk-wbt into rq_qos I
left the wbt_wait() where it was and simply copied and modified it
slightly to work for io-latency.  However they are both basically the
same thing, and as time has gone on wbt_wait() has ended up much smarter
and kinder than it was when I copied it into io-latency, which means
io-latency has lost out on these improvements.

Since they are the same thing essentially except for a few minor things,
create rq_qos_wait() that replicates what wbt_wait() currently does with
callbacks that can be passed in for the snowflakes to do their own thing
as appropriate.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 86 insertions, 0 deletions

diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c
index 80f603b76f61..e932ef9d2718 100644
--- a/block/blk-rq-qos.c
+++ b/block/blk-rq-qos.c
@@ -176,6 +176,92 @@ void rq_depth_scale_down(struct rq_depth *rqd, bool hard_throttle)
         rq_depth_calc_max_depth(rqd);
 }
 
+struct rq_qos_wait_data {
+        struct wait_queue_entry wq;
+        struct task_struct *task;
+        struct rq_wait *rqw;
+        acquire_inflight_cb_t *cb;
+        void *private_data;
+        bool got_token;
+};
+
+static int rq_qos_wake_function(struct wait_queue_entry *curr,
+                                unsigned int mode, int wake_flags, void *key)
+{
+        struct rq_qos_wait_data *data = container_of(curr,
+                                                     struct rq_qos_wait_data,
+                                                     wq);
+
+        /*
+         * If we fail to get a budget, return -1 to interrupt the wake up loop
+         * in __wake_up_common.
+         */
+        if (!data->cb(data->rqw, data->private_data))
+                return -1;
+
+        data->got_token = true;
+        list_del_init(&curr->entry);
+        wake_up_process(data->task);
+        return 1;
+}
+
+/**
+ * rq_qos_wait - throttle on a rqw if we need to
+ * @private_data - caller provided specific data
+ * @acquire_inflight_cb - inc the rqw->inflight counter if we can
+ * @cleanup_cb - the callback to cleanup in case we race with a waker
+ *
+ * This provides a uniform place for the rq_qos users to do their throttling.
+ * Since you can end up with a lot of things sleeping at once, this manages the
+ * waking up based on the resources available.  The acquire_inflight_cb should
+ * inc the rqw->inflight if we have the ability to do so, or return false if not
+ * and then we will sleep until the room becomes available.
+ *
+ * cleanup_cb is in case that we race with a waker and need to cleanup the
+ * inflight count accordingly.
+ */
+void rq_qos_wait(struct rq_wait *rqw, void *private_data,
+                 acquire_inflight_cb_t *acquire_inflight_cb,
+                 cleanup_cb_t *cleanup_cb)
+{
+        struct rq_qos_wait_data data = {
+                .wq = {
+                        .func   = rq_qos_wake_function,
+                        .entry  = LIST_HEAD_INIT(data.wq.entry),
+                },
+                .task = current,
+                .rqw = rqw,
+                .cb = acquire_inflight_cb,
+                .private_data = private_data,
+        };
+        bool has_sleeper;
+
+        has_sleeper = wq_has_sleeper(&rqw->wait);
+        if (!has_sleeper && acquire_inflight_cb(rqw, private_data))
+                return;
+
+        prepare_to_wait_exclusive(&rqw->wait, &data.wq, TASK_UNINTERRUPTIBLE);
+        do {
+                if (data.got_token)
+                        break;
+                if (!has_sleeper && acquire_inflight_cb(rqw, private_data)) {
+                        finish_wait(&rqw->wait, &data.wq);
+
+                        /*
+                         * We raced with wbt_wake_function() getting a token,
+                         * which means we now have two. Put our local token
+                         * and wake anyone else potentially waiting for one.
+                         */
+                        if (data.got_token)
+                                cleanup_cb(rqw, private_data);
+                        break;
+                }
+                io_schedule();
+                has_sleeper = false;
+        } while (1);
+        finish_wait(&rqw->wait, &data.wq);
+}
+
 void rq_qos_exit(struct request_queue *q)
 {
         while (q->rq_qos) {