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tcp: fix undo after RTO for CUBIC

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This patch fixes CUBIC so that cwnd reductions made during RTOs can be
undone (just as they already can be undone when using the default/Reno
behavior).

When undoing cwnd reductions, BIC-derived congestion control modules
were restoring the cwnd from last_max_cwnd. There were two problems
with using last_max_cwnd to restore a cwnd during undo:

(a) last_max_cwnd was set to 0 on state transitions into TCP_CA_Loss
(by calling the module's reset() functions), so cwnd reductions from
RTOs could not be undone.

(b) when fast_covergence is enabled (which it is by default)
last_max_cwnd does not actually hold the value of snd_cwnd before the
loss; instead, it holds a scaled-down version of snd_cwnd.

This patch makes the following changes:

(1) upon undo, revert snd_cwnd to ca->loss_cwnd, which is already, as
the existing comment notes, the "congestion window at last loss"

(2) stop forgetting ca->loss_cwnd on TCP_CA_Loss events

(3) use ca->last_max_cwnd to check if we're in slow start

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Acked-by: Sangtae Ha <sangtae.ha@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Neal Cardwell 2012-01-18 17:47:59 +00:00 committed by David S. Miller
parent fc16dcd8c2
commit 5a45f0086a
1 changed files with 6 additions and 4 deletions
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10
net/ipv4/tcp_cubic.c
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@ -107,7 +107,6 @@ static inline void bictcp_reset(struct bictcp *ca)
{
ca->cnt = 0;
ca->last_max_cwnd = 0;
ca->loss_cwnd = 0;
ca->last_cwnd = 0;
ca->last_time = 0;
ca->bic_origin_point = 0;
@ -142,7 +141,10 @@ static inline void bictcp_hystart_reset(struct sock *sk)
static void bictcp_init(struct sock *sk)
{
bictcp_reset(inet_csk_ca(sk));
struct bictcp *ca = inet_csk_ca(sk);
bictcp_reset(ca);
ca->loss_cwnd = 0;
if (hystart)
bictcp_hystart_reset(sk);
@ -275,7 +277,7 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
* The initial growth of cubic function may be too conservative
* when the available bandwidth is still unknown.
*/
if (ca->loss_cwnd == 0 && ca->cnt > 20)
if (ca->last_max_cwnd == 0 && ca->cnt > 20)
ca->cnt = 20; /* increase cwnd 5% per RTT */
/* TCP Friendly */
@ -342,7 +344,7 @@ static u32 bictcp_undo_cwnd(struct sock *sk)
{
struct bictcp *ca = inet_csk_ca(sk);
return max(tcp_sk(sk)->snd_cwnd, ca->last_max_cwnd);
return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd);
}
static void bictcp_state(struct sock *sk, u8 new_state)