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

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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>
This commit is contained in:
Manoj N. Kumar 2016-03-25 14:26:49 -05:00 committed by Martin K. Petersen
parent 9526f36026
commit ea76543127
1 changed files with 4 additions and 4 deletions
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8
drivers/scsi/cxlflash/main.c
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@ -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;
}