block: reimplement FLUSH/FUA to support merge

The current FLUSH/FUA support has evolved from the implementation
which had to perform queue draining.  As such, sequencing is done
queue-wide one flush request after another.  However, with the
draining requirement gone, there's no reason to keep the queue-wide
sequential approach.

This patch reimplements FLUSH/FUA support such that each FLUSH/FUA
request is sequenced individually.  The actual FLUSH execution is
double buffered and whenever a request wants to execute one for either
PRE or POSTFLUSH, it queues on the pending queue.  Once certain
conditions are met, a flush request is issued and on its completion
all pending requests proceed to the next sequence.

This allows arbitrary merging of different type of flushes.  How they
are merged can be primarily controlled and tuned by adjusting the
above said 'conditions' used to determine when to issue the next
flush.

This is inspired by Darrick's patches to merge multiple zero-data
flushes which helps workloads with highly concurrent fsync requests.

* As flush requests are never put on the IO scheduler, request fields
  used for flush share space with rq->rb_node.  rq->completion_data is
  moved out of the union.  This increases the request size by one
  pointer.

  As rq->elevator_private* are used only by the iosched too, it is
  possible to reduce the request size further.  However, to do that,
  we need to modify request allocation path such that iosched data is
  not allocated for flush requests.

* FLUSH/FUA processing happens on insertion now instead of dispatch.

- Comments updated as per Vivek and Mike.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: "Darrick J. Wong" <djwong@us.ibm.com>
Cc: Shaohua Li <shli@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

1 files changed, 12 insertions, 6 deletions

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 36ab42c9bb99..6d7e9afd08c3 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -99,13 +99,18 @@ struct request {
         /*
          * The rb_node is only used inside the io scheduler, requests
          * are pruned when moved to the dispatch queue. So let the
-         * completion_data share space with the rb_node.
+         * flush fields share space with the rb_node.
          */
         union {
                 struct rb_node rb_node; /* sort/lookup */
-                void *completion_data;
+                struct {
+                        unsigned int                    seq;
+                        struct list_head                list;
+                } flush;
         };
 
+        void *completion_data;
+
         /*
          * Three pointers are available for the IO schedulers, if they need
          * more they have to dynamically allocate it.
@@ -362,11 +367,12 @@ struct request_queue
          * for flush operations
          */
         unsigned int            flush_flags;
-        unsigned int            flush_seq;
-        int                     flush_err;
+        unsigned int            flush_pending_idx:1;
+        unsigned int            flush_running_idx:1;
+        unsigned long           flush_pending_since;
+        struct list_head        flush_queue[2];
+        struct list_head        flush_data_in_flight;
         struct request          flush_rq;
-        struct request          *orig_flush_rq;
-        struct list_head        pending_flushes;
 
         struct mutex            sysfs_lock;