writeback: eliminate work item allocation in bd_start_writeback()

Handle start-all writeback like we do periodic or kupdate
style writeback - by marking the bdi_writeback as needing a full
flush, and simply waking the thread. This eliminates the need to
allocate and queue a specific work item just for this purpose.

After this change, we truly only ever have one of them running at
any point in time. We mark the need to start all flushes, and the
writeback thread will clear it once it has processed the request.

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

2 files changed, 23 insertions, 22 deletions

diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index 420de5c7c7f9..b7c7be6f5986 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -45,6 +45,28 @@ enum wb_stat_item {
 #define WB_STAT_BATCH (8*(1+ilog2(nr_cpu_ids)))
 
 /*
+ * why some writeback work was initiated
+ */
+enum wb_reason {
+	WB_REASON_BACKGROUND,
+	WB_REASON_VMSCAN,
+	WB_REASON_SYNC,
+	WB_REASON_PERIODIC,
+	WB_REASON_LAPTOP_TIMER,
+	WB_REASON_FREE_MORE_MEM,
+	WB_REASON_FS_FREE_SPACE,
+	/*
+	 * There is no bdi forker thread any more and works are done
+	 * by emergency worker, however, this is TPs userland visible
+	 * and we'll be exposing exactly the same information,
+	 * so it has a mismatch name.
+	 */
+	WB_REASON_FORKER_THREAD,
+
+	WB_REASON_MAX,
+};
+
+/*
  * For cgroup writeback, multiple wb's may map to the same blkcg.  Those
  * wb's can operate mostly independently but should share the congested
  * state.  To facilitate such sharing, the congested state is tracked using
@@ -116,6 +138,7 @@ struct bdi_writeback {
 
 	struct fprop_local_percpu completions;
 	int dirty_exceeded;
+	enum wb_reason start_all_reason;
 
 	spinlock_t work_lock;		/* protects work_list & dwork scheduling */
 	struct list_head work_list;
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 9c0091678af4..dd1d2c23f743 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -42,28 +42,6 @@ enum writeback_sync_modes {
 };
 
 /*
- * why some writeback work was initiated
- */
-enum wb_reason {
-	WB_REASON_BACKGROUND,
-	WB_REASON_VMSCAN,
-	WB_REASON_SYNC,
-	WB_REASON_PERIODIC,
-	WB_REASON_LAPTOP_TIMER,
-	WB_REASON_FREE_MORE_MEM,
-	WB_REASON_FS_FREE_SPACE,
-	/*
-	 * There is no bdi forker thread any more and works are done
-	 * by emergency worker, however, this is TPs userland visible
-	 * and we'll be exposing exactly the same information,
-	 * so it has a mismatch name.
-	 */
-	WB_REASON_FORKER_THREAD,
-
-	WB_REASON_MAX,
-};
-
-/*
  * A control structure which tells the writeback code what to do.  These are
  * always on the stack, and hence need no locking.  They are always initialised
  * in a manner such that unspecified fields are set to zero.