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Subject[ 017/187] bql: Avoid unneeded limit decrement.
Date
From: Greg KH <gregkh@linuxfoundation.org>

3.4-stable review patch. If anyone has any objections, please let me know.

------------------


From: Hiroaki SHIMODA <shimoda.hiroaki@gmail.com>

[ 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 | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

--- 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, unsi
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)) {



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