Commit d4009e4b authored by Manoj N. Kumar's avatar Manoj N. Kumar Committed by Greg Kroah-Hartman

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

[ Upstream commit ea765431 ]

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: default avatarManoj N. Kumar <manoj@linux.vnet.ibm.com>
Acked-by: default avatarMatthew R. Ochs <mrochs@linux.vnet.ibm.com>
Reviewed-by: default avatarJohannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: default avatarUma Krishnan <ukrishn@linux.vnet.ibm.com>
Signed-off-by: default avatarMartin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: default avatarSasha Levin <alexander.levin@verizon.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 61610c63
......@@ -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;
}
......
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