[PATCH v2 2/2] cxlflash: Move to exponential back-off when cmd_room is not available

[Uma Krishnan <ukrishn@linux.vnet.ibm.com>]

From: "Manoj N. Kumar" <manoj@linux.vnet.ibm.com>

While profiling the cxlflash_queuecommand() path under a heavy load it
was found that number of retries to find cmd_room was fairly high.

There are two problems with the current back-off:
a) It starts with a udelay of 0
b) It backs-off linearly

Tried several approaches (a higher multiple 10*n, 100*n, as well as n^2,
2^n) and found that the exponential back-off(2^n) approach had the least
overall cost. Cost as being defined as overall time spent waiting.

The fix is to change the linear back-off to an exponential back-off.
This solution also takes care of the problem with the initial
delay (starts with 1 usec).

Signed-off-by: Manoj N. Kumar <manoj@linux.vnet.ibm.com>
Acked-by: Matthew R. Ochs <mrochs@linux.vnet.ibm.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Uma Krishnan <ukrishn@linux.vnet.ibm.com>
---
 drivers/scsi/cxlflash/main.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/scsi/cxlflash/main.c b/drivers/scsi/cxlflash/main.c
index e6d56ac..8fb9643 100644
--- a/drivers/scsi/cxlflash/main.c
+++ b/drivers/scsi/cxlflash/main.c
@@ -289,7 +289,7 @@ static void context_reset(struct afu_cmd *cmd)
 		atomic64_set(&afu->room, room);
 		if (room)
 			goto write_rrin;
-		udelay(nretry);
+		udelay(1 << nretry);
 	} while (nretry++ < MC_ROOM_RETRY_CNT);
 
 	pr_err("%s: no cmd_room to send reset\n", __func__);
@@ -303,7 +303,7 @@ write_rrin:
 		if (rrin != 0x1)
 			break;
 		/* Double delay each time */
-		udelay(2 << nretry);
+		udelay(1 << nretry);
 	} while (nretry++ < MC_ROOM_RETRY_CNT);
 }
 
@@ -338,7 +338,7 @@ retry:
 			atomic64_set(&afu->room, room);
 			if (room)
 				goto write_ioarrin;
-			udelay(nretry);
+			udelay(1 << nretry);
 		} while (nretry++ < MC_ROOM_RETRY_CNT);
 
 		dev_err(dev, "%s: no cmd_room to send 0x%X\n",
@@ -352,7 +352,7 @@ retry:
 		 * afu->room.
 		 */
 		if (nretry++ < MC_ROOM_RETRY_CNT) {
-			udelay(nretry);
+			udelay(1 << nretry);
 			goto retry;
 		}
 
-- 
2.1.0