From ea76543127da32dec28af0a13ea1b06625fc085e Mon Sep 17 00:00:00 2001 From: "Manoj N. Kumar" Date: Fri, 25 Mar 2016 14:26:49 -0500 Subject: [PATCH] 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 Acked-by: Matthew R. Ochs Reviewed-by: Johannes Thumshirn Signed-off-by: Uma Krishnan Signed-off-by: Martin K. Petersen --- 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 1d41807..724b771 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; } -- 2.7.4