netdev - [PATCH net 2/3] bql: Avoid unneeded limit decrement.

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Message-Id: <20120531072519.16464513.shimoda.hiroaki@gmail.com>
Date:	Thu, 31 May 2012 07:25:19 +0900
From:	Hiroaki SHIMODA <shimoda.hiroaki@...il.com>
To:	davem@...emloft.net
Cc:	therbert@...gle.com, eric.dumazet@...il.com, denys@...p.net.lb,
	netdev@...r.kernel.org
Subject: [PATCH net 2/3] bql: Avoid unneeded limit decrement.

When below pattern is observed,

                                               TIME
       dql_queued()         dql_completed()     |
      a) initial state                          |
                                                |
      b) X bytes queued                         V

      c) Y bytes queued
                           d) X bytes completed
      e) Z bytes queued
                           f) Y bytes completed

a) dql->limit has already some value and there is no in-flight packet.
b) X bytes queued.
c) Y bytes queued and excess limit.
d) X bytes completed and dql->prev_ovlimit is set and also
   dql->prev_num_queued is set Y.
e) Z bytes queued.
f) Y bytes completed. inprogress and prev_inprogress are true.

At f), according to the comment, all_prev_completed becomes
true and limit should be increased. But POSDIFF() ignores
(completed == dql->prev_num_queued) case, so limit is decreased.

Signed-off-by: Hiroaki SHIMODA <shimoda.hiroaki@...il.com>
Cc: Tom Herbert <therbert@...gle.com>
Cc: Eric Dumazet <eric.dumazet@...il.com>
Cc: Denys Fedoryshchenko <denys@...p.net.lb>
---
 lib/dynamic_queue_limits.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/lib/dynamic_queue_limits.c b/lib/dynamic_queue_limits.c
index c87eb76..0fafa77 100644
--- a/lib/dynamic_queue_limits.c
+++ b/lib/dynamic_queue_limits.c
@@ -11,12 +11,14 @@
 #include <linux/dynamic_queue_limits.h>
 
 #define POSDIFF(A, B) ((int)((A) - (B)) > 0 ? (A) - (B) : 0)
+#define AFTER_EQ(A, B) ((int)((A) - (B)) >= 0)
 
 /* Records completed count and recalculates the queue limit */
 void dql_completed(struct dql *dql, unsigned int count)
 {
 	unsigned int inprogress, prev_inprogress, limit;
-	unsigned int ovlimit, all_prev_completed, completed;
+	unsigned int ovlimit, completed;
+	bool all_prev_completed;
 
 	/* Can't complete more than what's in queue */
 	BUG_ON(count > dql->num_queued - dql->num_completed);
@@ -26,7 +28,7 @@ void dql_completed(struct dql *dql, unsigned int count)
 	ovlimit = POSDIFF(dql->num_queued - dql->num_completed, limit);
 	inprogress = dql->num_queued - completed;
 	prev_inprogress = dql->prev_num_queued - dql->num_completed;
-	all_prev_completed = POSDIFF(completed, dql->prev_num_queued);
+	all_prev_completed = AFTER_EQ(completed, dql->prev_num_queued);
 
 	if ((ovlimit && !inprogress) ||
 	    (dql->prev_ovlimit && all_prev_completed)) {
-- 
1.7.3.4

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