From: Mikulas Patocka <mpatocka@redhat.com>
Date: Tue, 19 Aug 2008 16:40:16 -0400
Subject: [block] performance fix for too many physical devices
Message-id: Pine.LNX.4.64.0808191414360.12962@hs20-bc2-1.build.redhat.com
O-Subject: [PATCH RHEL 5.3] bz459527 performance fix for too many physical devices
Bugzilla: 459527
RH-Acked-by: Milan Broz <mbroz@redhat.com>
RH-Acked-by: Jeff Moyer <jmoyer@redhat.com>

This patch improves performance of the block layer with too many physical
devices mapped via device mapper.

Testing: I compiled it on my workstation and tested it for correctness
(and tested that it doesn't damage performance). I didn't test performance
with that many devices, Mike Anderson did and there was overall 18%
improvement (of the whole benchmark, so the improvement in the block layer
is much more).

Upstream commit: dbaf2c003e151ad9231778819b0977f95e20e06f (2.6.26-rc2)

diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index 2828104..56ffc13 100644
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -1705,9 +1705,11 @@ EXPORT_SYMBOL(__generic_unplug_device);
  **/
 void generic_unplug_device(request_queue_t *q)
 {
-	spin_lock_irq(q->queue_lock);
-	__generic_unplug_device(q);
-	spin_unlock_irq(q->queue_lock);
+	if (blk_queue_plugged(q)) {
+		spin_lock_irq(q->queue_lock);
+		__generic_unplug_device(q);
+		spin_unlock_irq(q->queue_lock);
+	}
 }
 EXPORT_SYMBOL(generic_unplug_device);