hpsa: optimize cmd_alloc function by remembering last allocation

Empirically, this improves performance slightly (~2% max IOPS) by
allowing cmd_alloc to remember where it left off searching for
free commands between calls instead of always starting its search
at command 0.

Reviewed-by: Scott Teel <scott.teel@pmcs.com>
Signed-off-by: Robert Elliott <elliott@hp.com>
Signed-off-by: Don Brace <don.brace@pmcs.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

2 files changed, 6 insertions, 2 deletions

diff --git a/drivers/scsi/hpsa.c b/drivers/scsi/hpsa.c
index c95a20c5269b..72abcf3bfabf 100644
--- a/drivers/scsi/hpsa.c
+++ b/drivers/scsi/hpsa.c
@@ -4649,9 +4649,10 @@ static struct CommandList *cmd_alloc(struct ctlr_info *h)
         union u64bit temp64;
         dma_addr_t cmd_dma_handle, err_dma_handle;
         int refcount;
-        unsigned long offset = 0;
+        unsigned long offset;
 
-        /* There is some *extremely* small but non-zero chance that that
+        /*
+         * There is some *extremely* small but non-zero chance that that
          * multiple threads could get in here, and one thread could
          * be scanning through the list of bits looking for a free
          * one, but the free ones are always behind him, and other
@@ -4662,6 +4663,7 @@ static struct CommandList *cmd_alloc(struct ctlr_info *h)
          * infrequently as to be indistinguishable from never.
          */
 
+        offset = h->last_allocation; /* benignly racy */
         for (;;) {
                 i = find_next_zero_bit(h->cmd_pool_bits, h->nr_cmds, offset);
                 if (unlikely(i == h->nr_cmds)) {
@@ -4679,6 +4681,7 @@ static struct CommandList *cmd_alloc(struct ctlr_info *h)
                         h->cmd_pool_bits + (i / BITS_PER_LONG));
                 break; /* it's ours now. */
         }
+        h->last_allocation = i; /* benignly racy */
 
         /* Zero out all of commandlist except the last field, refcount */
         memset(c, 0, offsetof(struct CommandList, refcount));
diff --git a/drivers/scsi/hpsa.h b/drivers/scsi/hpsa.h
index 679e4d2272e0..981479a13935 100644
--- a/drivers/scsi/hpsa.h
+++ b/drivers/scsi/hpsa.h
@@ -133,6 +133,7 @@ struct ctlr_info {
         struct CfgTable __iomem *cfgtable;
         int     interrupts_enabled;
         int     max_commands;
+        int last_allocation;
         atomic_t commands_outstanding;
 #       define PERF_MODE_INT    0
 #       define DOORBELL_INT     1