bql: Avoid unneeded limit decrement.
Hiroaki SHIMODA [Wed, 30 May 2012 12:25:19 +0000 (12:25 +0000)]
[ Upstream commit 25426b794efdc70dde7fd3134dc56fac3e7d562d ]

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@gmail.com>
Cc: Tom Herbert <therbert@google.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Denys Fedoryshchenko <denys@visp.net.lb>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

lib/dynamic_queue_limits.c

index c87eb76..0fafa77 100644 (file)
 #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)) {