scsi: target: Fix write perf due to unneeded throttling

The write back throttling (WBT) code checks if REQ_SYNC | REQ_IDLE is set
to determine if a write is O_DIRECT vs buffered. If the bits are not set
then it assumes it's a buffered write and will throttle LIO if we hit
certain metrics. LIO itself is not using the buffer cache and is doing
direct I/O, so this has us set the direct bits so we are not throttled.

When the initiator application is doing direct I/O this can greatly improve
performance. It depends on the backend device but we have seen where the
WBT code is throttling writes to only 20K IOPs with 4K I/Os when the device
can support 100K+.

Signed-off-by: Mike Christie <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Martin K. Petersen <[email protected]>

Author: mikechristie

drivers/target/target_core_iblock.c

@@ -740,11 +740,16 @@ iblock_execute_rw(struct se_cmd *cmd, struct scatterlist *sgl, u32 sgl_nents,
 
 	if (data_direction == DMA_TO_DEVICE) {
 		struct iblock_dev *ib_dev = IBLOCK_DEV(dev);
+
+		/*
+		 * Set bits to indicate WRITE_ODIRECT so we are not throttled
+		 * by WBT.
+		 */
+		opf = REQ_OP_WRITE | REQ_SYNC | REQ_IDLE;
 		/*
 		 * Force writethrough using REQ_FUA if a volatile write cache
 		 * is not enabled, or if initiator set the Force Unit Access bit.
 		 */
-		opf = REQ_OP_WRITE;
 		miter_dir = SG_MITER_TO_SG;
 		if (bdev_fua(ib_dev->ibd_bd)) {
 			if (cmd->se_cmd_flags & SCF_FUA)