cxlflash: Move to exponential back-off when cmd_room is not available

[ Upstream commit ea76543127da32dec28af0a13ea1b06625fc085e ]

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>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/scsi/cxlflash/main.c b/drivers/scsi/cxlflash/main.c
index 75ff7bd..c86847c 100644 (file)
--- 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;
                }