[PATCH 2/2] scsi: target: tcm_loop: use scsi_done_direct()

[Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>]

Tcm_loop uses workqueue to end requests, which is non-interrupt context,
then we can complete request directly instead deferring it to softirq.
The call graph likes below:
    blk_mq_complete_request_remote+1
    blk_mq_complete_request+14
    target_put_sess_cmd+294
    transport_generic_free_cmd+93
    target_complete_ok_work+251
    process_one_work+482
    worker_thread+80
    kthread+361
    ret_from_fork+31

Use tcm_loop and tcmu(backstore is file) to evaluate performance, fio
job:
  [global]
  filename=/dev/sdb
  direct=1
  runtime=30
  thread=1
  norandommap=1
  time_based
  numjobs=1
  rw=randread
  iodepth=32
  ioengine=libaio

Without this patch:
bs 4k
  READ: bw=319MiB/s (334MB/s), 319MiB/s-319MiB/s (334MB/s-334MB/s),
io=9563MiB (10.0GB), run=30001-30001msec

bs 8k:
   READ: bw=611MiB/s (641MB/s), 611MiB/s-611MiB/s (641MB/s-641MB/s),
io=17.9GiB (19.2GB), run=30001-30001msec

bs 16k:
   READ: bw=1109MiB/s (1163MB/s), 1109MiB/s-1109MiB/s (1163MB/s-1163MB/s),
io=32.5GiB (34.9GB), run=30001-30001msec

bs 32k:
   READ: bw=2200MiB/s (2306MB/s), 2200MiB/s-2200MiB/s (2306MB/s-2306MB/s),
io=64.4GiB (69.2GB), run=30001-30001msec

With this patch:
bs 4k:
   READ: bw=344MiB/s (361MB/s), 344MiB/s-344MiB/s (361MB/s-361MB/s),
io=10.1GiB (10.8GB), run=30001-30001msec

bs 8k:
   READ: bw=651MiB/s (682MB/s), 651MiB/s-651MiB/s (682MB/s-682MB/s),
io=19.1GiB (20.5GB), run=30001-30001msec

bs 16k:
   READ: bw=1248MiB/s (1308MB/s), 1248MiB/s-1248MiB/s (1308MB/s-1308MB/s),
io=36.6GiB (39.3GB), run=30001-30001msec

bs 32k:
   READ: bw=2456MiB/s (2576MB/s), 2456MiB/s-2456MiB/s (2576MB/s-2576MB/s),
io=71.0GiB (77.3GB), run=30001-30001msec

We can get throughput improvement.

Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
---
 drivers/target/loopback/tcm_loop.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/target/loopback/tcm_loop.c b/drivers/target/loopback/tcm_loop.c
index 4407b56aa6d1..ce414fbdbae6 100644
--- a/drivers/target/loopback/tcm_loop.c
+++ b/drivers/target/loopback/tcm_loop.c
@@ -70,8 +70,12 @@ static void tcm_loop_release_cmd(struct se_cmd *se_cmd)
 
 	if (se_cmd->se_cmd_flags & SCF_SCSI_TMR_CDB)
 		kmem_cache_free(tcm_loop_cmd_cache, tl_cmd);
-	else
-		scsi_done(sc);
+	else {
+		if (unlikely(in_interrupt()))
+			scsi_done(sc);
+		else
+			scsi_done_direct(sc);
+	}
 }
 
 static int tcm_loop_show_info(struct seq_file *m, struct Scsi_Host *host)
-- 
2.14.4.44.g2045bb6