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codel: refine one condition to avoid a nul rec_inv_sqrt 

One condition before codel_Newton_step() was not good if
we never left the dropping state for a flow. As a result
rec_inv_sqrt was 0, instead of the ~0 initial value.

codel control law was then set to a very aggressive mode, dropping
many packets before reaching 'target' and recovering from this problem.

To keep codel_vars_init() as efficient as possible, refine
the condition to make sure rec_inv_sqrt initial value is correct

Many thanks to Anton Mich for discovering the issue and suggesting
a fix.

Reported-by: Anton Mich <lp2s1h@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2012-07-29 20:52:21 +00:00 committed by David S. Miller
parent 55461ddbcb
commit 2359a47671
1 changed files with 6 additions and 2 deletions
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8
include/net/codel.h
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@ -305,6 +305,8 @@ static struct sk_buff *codel_dequeue(struct Qdisc *sch,
}
}
} else if (drop) {
u32 delta;
if (params->ecn && INET_ECN_set_ce(skb)) {
stats->ecn_mark++;
} else {
@ -320,9 +322,11 @@ static struct sk_buff *codel_dequeue(struct Qdisc *sch,
* assume that the drop rate that controlled the queue on the
* last cycle is a good starting point to control it now.
*/
if (codel_time_before(now - vars->drop_next,
delta = vars->count - vars->lastcount;
if (delta > 1 &&
codel_time_before(now - vars->drop_next,
16 * params->interval)) {
vars->count = (vars->count - vars->lastcount) | 1;
vars->count = delta;
/* we dont care if rec_inv_sqrt approximation
* is not very precise :
* Next Newton steps will correct it quadratically.