Commit ea765431 authored Mar 25, 2016 by Manoj N. Kumar Committed by Martin K. Petersen Mar 28, 2016

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



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>

parent 9526f360

drivers/scsi/cxlflash/main.c

+4 −4

Original line number	Diff line number	Diff line
		@@ -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 @@ static void context_reset(struct afu_cmd *cmd)
		if (rrin != 0x1)
		break;
		/* Double delay each time */
		udelay(2 << nretry);
		udelay(1 << nretry);
		} while (nretry++ < MC_ROOM_RETRY_CNT);
		}

		@@ -338,7 +338,7 @@ static int send_cmd(struct afu afu, struct afu_cmd cmd)
		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 @@ static int send_cmd(struct afu afu, struct afu_cmd cmd)
		* afu->room.
		*/
		if (nretry++ < MC_ROOM_RETRY_CNT) {
		udelay(nretry);
		udelay(1 << nretry);
		goto retry;
		}