[BLOCK] Reimplement elevator switch

 This patch reimplements elevator switch.  This patch assumes generic
dispatch queue patchset is applied.

 * Each request is tagged with REQ_ELVPRIV flag if it has its elevator
   private data set.
 * Requests which doesn't have REQ_ELVPRIV flag set never enter
   iosched.  They are always directly back inserted to dispatch queue.
   Of course, elevator_put_req_fn is called only for requests which
   have its REQ_ELVPRIV set.
 * Request queue maintains the current number of requests which have
   its elevator data set (elevator_set_req_fn called) in
   q->rq->elvpriv.
 * If a request queue has QUEUE_FLAG_BYPASS set, elevator private data
   is not allocated for new requests.

 To switch to another iosched, we set QUEUE_FLAG_BYPASS and wait until
elvpriv goes to zero; then, we attach the new iosched and clears
QUEUE_FLAG_BYPASS.  New implementation is much simpler and main code
paths are less cluttered, IMHO.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jens Axboe <axboe@suse.de>

2 files changed, 62 insertions, 158 deletions

diff --git a/drivers/block/elevator.c b/drivers/block/elevator.c
index af2388e73f61..272d93946621 100644
--- a/drivers/block/elevator.c
+++ b/drivers/block/elevator.c
@@ -34,6 +34,7 @@
 #include <linux/slab.h>
 #include <linux/init.h>
 #include <linux/compiler.h>
+#include <linux/delay.h>
 
 #include <asm/uaccess.h>
 
@@ -131,11 +132,7 @@ static int elevator_attach(request_queue_t *q, struct elevator_type *e,
         eq->ops = &e->ops;
         eq->elevator_type = e;
 
-        INIT_LIST_HEAD(&q->queue_head);
-        q->last_merge = NULL;
         q->elevator = eq;
-        q->end_sector = 0;
-        q->boundary_rq = NULL;
 
         if (eq->ops->elevator_init_fn)
                 ret = eq->ops->elevator_init_fn(q, eq);
@@ -184,6 +181,12 @@ int elevator_init(request_queue_t *q, char *name)
         struct elevator_queue *eq;
         int ret = 0;
 
+        INIT_LIST_HEAD(&q->queue_head);
+        q->last_merge = NULL;
+        q->end_sector = 0;
+        q->boundary_rq = NULL;
+        q->max_back_kb = 0;
+
         elevator_setup_default();
 
         if (!name)
@@ -336,23 +339,14 @@ void __elv_add_request(request_queue_t *q, struct request *rq, int where,
                         q->end_sector = rq_end_sector(rq);
                         q->boundary_rq = rq;
                 }
-        }
+        } else if (!(rq->flags & REQ_ELVPRIV) && where == ELEVATOR_INSERT_SORT)
+                where = ELEVATOR_INSERT_BACK;
 
         if (plug)
                 blk_plug_device(q);
 
         rq->q = q;
 
-        if (unlikely(test_bit(QUEUE_FLAG_DRAIN, &q->queue_flags))) {
-                /*
-                 * if drain is set, store the request "locally". when the drain
-                 * is finished, the requests will be handed ordered to the io
-                 * scheduler
-                 */
-                list_add_tail(&rq->queuelist, &q->drain_list);
-                return;
-        }
-
         switch (where) {
         case ELEVATOR_INSERT_FRONT:
                 rq->flags |= REQ_SOFTBARRIER;
@@ -659,25 +653,36 @@ EXPORT_SYMBOL_GPL(elv_unregister);
  * switch to new_e io scheduler. be careful not to introduce deadlocks -
  * we don't free the old io scheduler, before we have allocated what we
  * need for the new one. this way we have a chance of going back to the old
- * one, if the new one fails init for some reason. we also do an intermediate
- * switch to noop to ensure safety with stack-allocated requests, since they
- * don't originate from the block layer allocator. noop is safe here, because
- * it never needs to touch the elevator itself for completion events. DRAIN
- * flags will make sure we don't touch it for additions either.
+ * one, if the new one fails init for some reason.
  */
 static void elevator_switch(request_queue_t *q, struct elevator_type *new_e)
 {
-        elevator_t *e = kmalloc(sizeof(elevator_t), GFP_KERNEL);
-        struct elevator_type *noop_elevator = NULL;
-        elevator_t *old_elevator;
+        elevator_t *old_elevator, *e;
 
+        /*
+         * Allocate new elevator
+         */
+        e = kmalloc(sizeof(elevator_t), GFP_KERNEL);
         if (!e)
                 goto error;
 
         /*
-         * first step, drain requests from the block freelist
+         * Turn on BYPASS and drain all requests w/ elevator private data
          */
-        blk_wait_queue_drained(q, 0);
+        spin_lock_irq(q->queue_lock);
+
+        set_bit(QUEUE_FLAG_BYPASS, &q->queue_flags);
+
+        while (q->elevator->ops->elevator_dispatch_fn(q, 1))
+                ;
+
+        while (q->rq.elvpriv) {
+                spin_unlock_irq(q->queue_lock);
+                msleep(100);
+                spin_lock_irq(q->queue_lock);
+        }
+
+        spin_unlock_irq(q->queue_lock);
 
         /*
          * unregister old elevator data
@@ -686,18 +691,6 @@ static void elevator_switch(request_queue_t *q, struct elevator_type *new_e)
         old_elevator = q->elevator;
 
         /*
-         * next step, switch to noop since it uses no private rq structures
-         * and doesn't allocate any memory for anything. then wait for any
-         * non-fs requests in-flight
-         */
-        noop_elevator = elevator_get("noop");
-        spin_lock_irq(q->queue_lock);
-        elevator_attach(q, noop_elevator, e);
-        spin_unlock_irq(q->queue_lock);
-
-        blk_wait_queue_drained(q, 1);
-
-        /*
          * attach and start new elevator
          */
         if (elevator_attach(q, new_e, e))
@@ -707,11 +700,10 @@ static void elevator_switch(request_queue_t *q, struct elevator_type *new_e)
                 goto fail_register;
 
         /*
-         * finally exit old elevator and start queue again
+         * finally exit old elevator and turn off BYPASS.
          */
         elevator_exit(old_elevator);
-        blk_finish_queue_drain(q);
-        elevator_put(noop_elevator);
+        clear_bit(QUEUE_FLAG_BYPASS, &q->queue_flags);
         return;
 
 fail_register:
@@ -720,13 +712,13 @@ fail_register:
          * one again (along with re-adding the sysfs dir)
          */
         elevator_exit(e);
+        e = NULL;
 fail:
         q->elevator = old_elevator;
         elv_register_queue(q);
-        blk_finish_queue_drain(q);
+        clear_bit(QUEUE_FLAG_BYPASS, &q->queue_flags);
+        kfree(e);
 error:
-        if (noop_elevator)
-                elevator_put(noop_elevator);
         elevator_put(new_e);
         printk(KERN_ERR "elevator: switch to %s failed\n",new_e->elevator_name);
 }
diff --git a/drivers/block/ll_rw_blk.c b/drivers/block/ll_rw_blk.c
index d2a66fd309c3..f7c9931cb380 100644
--- a/drivers/block/ll_rw_blk.c
+++ b/drivers/block/ll_rw_blk.c
@@ -263,8 +263,6 @@ void blk_queue_make_request(request_queue_t * q, make_request_fn * mfn)
         blk_queue_bounce_limit(q, BLK_BOUNCE_HIGH);
 
         blk_queue_activity_fn(q, NULL, NULL);
-
-        INIT_LIST_HEAD(&q->drain_list);
 }
 
 EXPORT_SYMBOL(blk_queue_make_request);
@@ -1050,6 +1048,7 @@ static char *rq_flags[] = {
         "REQ_STARTED",
         "REQ_DONTPREP",
         "REQ_QUEUED",
+        "REQ_ELVPRIV",
         "REQ_PC",
         "REQ_BLOCK_PC",
         "REQ_SENSE",
@@ -1640,9 +1639,9 @@ static int blk_init_free_list(request_queue_t *q)
 
         rl->count[READ] = rl->count[WRITE] = 0;
         rl->starved[READ] = rl->starved[WRITE] = 0;
+        rl->elvpriv = 0;
         init_waitqueue_head(&rl->wait[READ]);
         init_waitqueue_head(&rl->wait[WRITE]);
-        init_waitqueue_head(&rl->drain);
 
         rl->rq_pool = mempool_create_node(BLKDEV_MIN_RQ, mempool_alloc_slab,
                                 mempool_free_slab, request_cachep, q->node);
@@ -1785,12 +1784,14 @@ EXPORT_SYMBOL(blk_get_queue);
 
 static inline void blk_free_request(request_queue_t *q, struct request *rq)
 {
-        elv_put_request(q, rq);
+        if (rq->flags & REQ_ELVPRIV)
+                elv_put_request(q, rq);
         mempool_free(rq, q->rq.rq_pool);
 }
 
 static inline struct request *
-blk_alloc_request(request_queue_t *q, int rw, struct bio *bio, int gfp_mask)
+blk_alloc_request(request_queue_t *q, int rw, struct bio *bio,
+                  int priv, int gfp_mask)
 {
         struct request *rq = mempool_alloc(q->rq.rq_pool, gfp_mask);
 
@@ -1803,11 +1804,15 @@ blk_alloc_request(request_queue_t *q, int rw, struct bio *bio, int gfp_mask)
          */
         rq->flags = rw;
 
-        if (!elv_set_request(q, rq, bio, gfp_mask))
-                return rq;
+        if (priv) {
+                if (unlikely(elv_set_request(q, rq, bio, gfp_mask))) {
+                        mempool_free(rq, q->rq.rq_pool);
+                        return NULL;
+                }
+                rq->flags |= REQ_ELVPRIV;
+        }
 
-        mempool_free(rq, q->rq.rq_pool);
-        return NULL;
+        return rq;
 }
 
 /*
@@ -1863,22 +1868,18 @@ static void __freed_request(request_queue_t *q, int rw)
  * A request has just been released.  Account for it, update the full and
  * congestion status, wake up any waiters.   Called under q->queue_lock.
  */
-static void freed_request(request_queue_t *q, int rw)
+static void freed_request(request_queue_t *q, int rw, int priv)
 {
         struct request_list *rl = &q->rq;
 
         rl->count[rw]--;
+        if (priv)
+                rl->elvpriv--;
 
         __freed_request(q, rw);
 
         if (unlikely(rl->starved[rw ^ 1]))
                 __freed_request(q, rw ^ 1);
-
-        if (!rl->count[READ] && !rl->count[WRITE]) {
-                smp_mb();
-                if (unlikely(waitqueue_active(&rl->drain)))
-                        wake_up(&rl->drain);
-        }
 }
 
 #define blkdev_free_rq(list) list_entry((list)->next, struct request, queuelist)
@@ -1893,9 +1894,7 @@ static struct request *get_request(request_queue_t *q, int rw, struct bio *bio,
         struct request *rq = NULL;
         struct request_list *rl = &q->rq;
         struct io_context *ioc = current_io_context(GFP_ATOMIC);
-
-        if (unlikely(test_bit(QUEUE_FLAG_DRAIN, &q->queue_flags)))
-                goto out;
+        int priv;
 
         if (rl->count[rw]+1 >= q->nr_requests) {
                 /*
@@ -1940,9 +1939,14 @@ get_rq:
         rl->starved[rw] = 0;
         if (rl->count[rw] >= queue_congestion_on_threshold(q))
                 set_queue_congested(q, rw);
+
+        priv = !test_bit(QUEUE_FLAG_BYPASS, &q->queue_flags);
+        if (priv)
+                rl->elvpriv++;
+
         spin_unlock_irq(q->queue_lock);
 
-        rq = blk_alloc_request(q, rw, bio, gfp_mask);
+        rq = blk_alloc_request(q, rw, bio, priv, gfp_mask);
         if (!rq) {
                 /*
                  * Allocation failed presumably due to memory. Undo anything
@@ -1952,7 +1956,7 @@ get_rq:
                  * wait queue, but this is pretty rare.
                  */
                 spin_lock_irq(q->queue_lock);
-                freed_request(q, rw);
+                freed_request(q, rw, priv);
 
                 /*
                  * in the very unlikely event that allocation failed and no
@@ -2470,11 +2474,12 @@ static void __blk_put_request(request_queue_t *q, struct request *req)
          */
         if (rl) {
                 int rw = rq_data_dir(req);
+                int priv = req->flags & REQ_ELVPRIV;
 
                 BUG_ON(!list_empty(&req->queuelist));
 
                 blk_free_request(q, req);
-                freed_request(q, rw);
+                freed_request(q, rw, priv);
         }
 }
 
@@ -2802,97 +2807,6 @@ static inline void blk_partition_remap(struct bio *bio)
         }
 }
 
-void blk_finish_queue_drain(request_queue_t *q)
-{
-        struct request_list *rl = &q->rq;
-        struct request *rq;
-        int requeued = 0;
-
-        spin_lock_irq(q->queue_lock);
-        clear_bit(QUEUE_FLAG_DRAIN, &q->queue_flags);
-
-        while (!list_empty(&q->drain_list)) {
-                rq = list_entry_rq(q->drain_list.next);
-
-                list_del_init(&rq->queuelist);
-                elv_requeue_request(q, rq);
-                requeued++;
-        }
-
-        if (requeued)
-                q->request_fn(q);
-
-        spin_unlock_irq(q->queue_lock);
-
-        wake_up(&rl->wait[0]);
-        wake_up(&rl->wait[1]);
-        wake_up(&rl->drain);
-}
-
-static int wait_drain(request_queue_t *q, struct request_list *rl, int dispatch)
-{
-        int wait = rl->count[READ] + rl->count[WRITE];
-
-        if (dispatch)
-                wait += !list_empty(&q->queue_head);
-
-        return wait;
-}
-
-/*
- * We rely on the fact that only requests allocated through blk_alloc_request()
- * have io scheduler private data structures associated with them. Any other
- * type of request (allocated on stack or through kmalloc()) should not go
- * to the io scheduler core, but be attached to the queue head instead.
- */
-void blk_wait_queue_drained(request_queue_t *q, int wait_dispatch)
-{
-        struct request_list *rl = &q->rq;
-        DEFINE_WAIT(wait);
-
-        spin_lock_irq(q->queue_lock);
-        set_bit(QUEUE_FLAG_DRAIN, &q->queue_flags);
-
-        while (wait_drain(q, rl, wait_dispatch)) {
-                prepare_to_wait(&rl->drain, &wait, TASK_UNINTERRUPTIBLE);
-
-                if (wait_drain(q, rl, wait_dispatch)) {
-                        __generic_unplug_device(q);
-                        spin_unlock_irq(q->queue_lock);
-                        io_schedule();
-                        spin_lock_irq(q->queue_lock);
-                }
-
-                finish_wait(&rl->drain, &wait);
-        }
-
-        spin_unlock_irq(q->queue_lock);
-}
-
-/*
- * block waiting for the io scheduler being started again.
- */
-static inline void block_wait_queue_running(request_queue_t *q)
-{
-        DEFINE_WAIT(wait);
-
-        while (unlikely(test_bit(QUEUE_FLAG_DRAIN, &q->queue_flags))) {
-                struct request_list *rl = &q->rq;
-
-                prepare_to_wait_exclusive(&rl->drain, &wait,
-                                TASK_UNINTERRUPTIBLE);
-
-                /*
-                 * re-check the condition. avoids using prepare_to_wait()
-                 * in the fast path (queue is running)
-                 */
-                if (test_bit(QUEUE_FLAG_DRAIN, &q->queue_flags))
-                        io_schedule();
-
-                finish_wait(&rl->drain, &wait);
-        }
-}
-
 static void handle_bad_sector(struct bio *bio)
 {
         char b[BDEVNAME_SIZE];
@@ -2988,8 +2902,6 @@ end_io:
                 if (unlikely(test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)))
                         goto end_io;
 
-                block_wait_queue_running(q);
-
                 /*
                  * If this device has partitions, remap block n
                  * of partition p to block n+start(p) of the disk.