[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>

1 files changed, 4 insertions, 6 deletions

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 159dbcd2eb59..6186d5e2110f 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -107,9 +107,9 @@ typedef void (rq_end_io_fn)(struct request *);
 struct request_list {
         int count[2];
         int starved[2];
+        int elvpriv;
         mempool_t *rq_pool;
         wait_queue_head_t wait[2];
-        wait_queue_head_t drain;
 };
 
 #define BLK_MAX_CDB     16
@@ -211,6 +211,7 @@ enum rq_flag_bits {
         __REQ_STARTED,          /* drive already may have started this one */
         __REQ_DONTPREP,         /* don't call prep for this one */
         __REQ_QUEUED,           /* uses queueing */
+        __REQ_ELVPRIV,          /* elevator private data attached */
         /*
          * for ATA/ATAPI devices
          */
@@ -244,6 +245,7 @@ enum rq_flag_bits {
 #define REQ_STARTED     (1 << __REQ_STARTED)
 #define REQ_DONTPREP    (1 << __REQ_DONTPREP)
 #define REQ_QUEUED      (1 << __REQ_QUEUED)
+#define REQ_ELVPRIV     (1 << __REQ_ELVPRIV)
 #define REQ_PC          (1 << __REQ_PC)
 #define REQ_BLOCK_PC    (1 << __REQ_BLOCK_PC)
 #define REQ_SENSE       (1 << __REQ_SENSE)
@@ -413,8 +415,6 @@ struct request_queue
         unsigned int            sg_reserved_size;
         int                     node;
 
-        struct list_head        drain_list;
-
         /*
          * reserved for flush operations
          */
@@ -442,7 +442,7 @@ enum {
 #define QUEUE_FLAG_DEAD         5       /* queue being torn down */
 #define QUEUE_FLAG_REENTER      6       /* Re-entrancy avoidance */
 #define QUEUE_FLAG_PLUGGED      7       /* queue is plugged */
-#define QUEUE_FLAG_DRAIN        8       /* draining queue for sched switch */
+#define QUEUE_FLAG_BYPASS       8       /* don't use elevator, just do FIFO */
 #define QUEUE_FLAG_FLUSH        9       /* doing barrier flush sequence */
 
 #define blk_queue_plugged(q)    test_bit(QUEUE_FLAG_PLUGGED, &(q)->queue_flags)
@@ -668,8 +668,6 @@ extern void blk_dump_rq_flags(struct request *, char *);
 extern void generic_unplug_device(request_queue_t *);
 extern void __generic_unplug_device(request_queue_t *);
 extern long nr_blockdev_pages(void);
-extern void blk_wait_queue_drained(request_queue_t *, int);
-extern void blk_finish_queue_drain(request_queue_t *);
 
 int blk_get_queue(request_queue_t *);
 request_queue_t *blk_alloc_queue(int gfp_mask);