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Message-ID: <CAK6E8=fNk-x6=QLHT0rjFRh3bARfkErA+VW_PgxRUcu-3fm9pQ@mail.gmail.com>
Date: Thu, 27 Aug 2015 15:57:35 -0700
From: Yuchung Cheng <ycheng@...gle.com>
To: Lawrence Brakmo <brakmo@...com>
Cc: netdev <netdev@...r.kernel.org>, Kernel Team <Kernel-team@...com>,
Neal Cardwell <ncardwell@...gle.com>,
Eric Dumazet <eric.dumazet@...il.com>,
Stephen Hemminger <stephen@...workplumber.org>,
Kenneth Klette Jonassen <kennetkl@....uio.no>
Subject: Re: [RFC PATCH v6 net-next 3/4] tcp: add in_flight to tcp_skb_cb
On Thu, Aug 27, 2015 at 3:54 PM, Yuchung Cheng <ycheng@...gle.com> wrote:
> On Thu, Aug 27, 2015 at 3:44 PM, Lawrence Brakmo <brakmo@...com> wrote:
>> Yuchung, thank you for reviewing these patches. Response inline below.
>>
>> On 8/27/15, 3:00 PM, "Yuchung Cheng" <ycheng@...gle.com> wrote:
>>
>>>On Tue, Aug 25, 2015 at 4:33 PM, Lawrence Brakmo <brakmo@...com> wrote:
>>>> Add in_flight (bytes in flight when packet was sent) field
>>>> to tx component of tcp_skb_cb and make it available to
>>>> congestion modules' pkts_acked() function through the
>>>> ack_sample function argument.
>>>>
>>>> Signed-off-by: Lawrence Brakmo <brakmo@...com>
> Acked-by: Yuchung Cheng <ycheng@...gle.com>
>
>>>> ---
>>>> include/net/tcp.h | 2 ++
>>>> net/ipv4/tcp_input.c | 5 ++++-
>>>> net/ipv4/tcp_output.c | 4 +++-
>>>> 3 files changed, 9 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/include/net/tcp.h b/include/net/tcp.h
>>>> index a086a98..cdd93e5 100644
>>>> --- a/include/net/tcp.h
>>>> +++ b/include/net/tcp.h
>>>> @@ -757,6 +757,7 @@ struct tcp_skb_cb {
>>>> union {
>>>> struct {
>>>> /* There is space for up to 20 bytes */
>>>> + __u32 in_flight;/* Bytes in flight when packet
>>>>sent */
>>>> } tx; /* only used for outgoing skbs */
>>>> union {
>>>> struct inet_skb_parm h4;
>>>> @@ -842,6 +843,7 @@ union tcp_cc_info;
>>>> struct ack_sample {
>>>> u32 pkts_acked;
>>>> s32 rtt_us;
>>>> + u32 in_flight;
>>>> };
>>>>
>>>> struct tcp_congestion_ops {
>>>> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
>>>> index f506a0a..338e6bb 100644
>>>> --- a/net/ipv4/tcp_input.c
>>>> +++ b/net/ipv4/tcp_input.c
>>>> @@ -3069,6 +3069,7 @@ static int tcp_clean_rtx_queue(struct sock *sk,
>>>>int prior_fackets,
>>>> long ca_rtt_us = -1L;
>>>> struct sk_buff *skb;
>>>> u32 pkts_acked = 0;
>>>> + u32 last_in_flight = 0;
>>>> bool rtt_update;
>>>> int flag = 0;
>>>>
>>>> @@ -3108,6 +3109,7 @@ static int tcp_clean_rtx_queue(struct sock *sk,
>>>>int prior_fackets,
>>>> if (!first_ackt.v64)
>>>> first_ackt = last_ackt;
>>>>
>>>> + last_in_flight = TCP_SKB_CB(skb)->tx.in_flight;
>>>> reord = min(pkts_acked, reord);
>>>> if (!after(scb->end_seq, tp->high_seq))
>>>> flag |= FLAG_ORIG_SACK_ACKED;
>>>> @@ -3197,7 +3199,8 @@ static int tcp_clean_rtx_queue(struct sock *sk,
>>>>int prior_fackets,
>>>> }
>>>>
>>>> if (icsk->icsk_ca_ops->pkts_acked) {
>>>> - struct ack_sample sample = {pkts_acked, ca_rtt_us};
>>>> + struct ack_sample sample = {pkts_acked, ca_rtt_us,
>>>> + last_in_flight};
>>>>
>>>> icsk->icsk_ca_ops->pkts_acked(sk, &sample);
>>>> }
>>>> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
>>>> index 444ab5b..244d201 100644
>>>> --- a/net/ipv4/tcp_output.c
>>>> +++ b/net/ipv4/tcp_output.c
>>>> @@ -920,9 +920,12 @@ static int tcp_transmit_skb(struct sock *sk,
>>>>struct sk_buff *skb, int clone_it,
>>>> int err;
>>>>
>>>> BUG_ON(!skb || !tcp_skb_pcount(skb));
>>>> + tp = tcp_sk(sk);
>>>>
>>>> if (clone_it) {
>>>> skb_mstamp_get(&skb->skb_mstamp);
>>>> + TCP_SKB_CB(skb)->tx.in_flight = TCP_SKB_CB(skb)->end_seq
>>>> + - tp->snd_una;
>>>what if skb is a retransmitted packet? e.g. the first retransmission
>>>in fast recovery would always record an inflight of 1 packet?
>>
>> Yes.
>> This does not affect NV for 2 reasons: 1) NV does not use ACKs when
>> ca_state is not Open or Disorder to determine congestion state, 2) even if
>> we used it, the small inflight means that the computed throughput will be
>> small so it will not cause a non-congestion signal, but will not cause a
>> congestion signal either because NV needs many (~60) measurements before
>> determining there is congestion.
>>
>> However, other consumers may prefer a different value. From a congestion
>> avoidance perspective, it is unclear we will be able to compute an
>> accurate throughput when retransmitting, so we may as well give a lower
>> bound.
> I see. Then this is OK for now since only NV uses it. We can enhance
> and track tput even during other CA states later. Would that be a
> useful feature for NV as well?
For example, we (at Google servers) have seen some flows staying in
very long CA_Recovery due to rate limiter or CA_Disorder state due to
high path reordering. It'd be beneficial to have CC continue to
operate in these circumstances in the future.
>
>>
>> What do you think?
>>
>>>
>>>>
>>>> if (unlikely(skb_cloned(skb)))
>>>> skb = pskb_copy(skb, gfp_mask);
>>>> @@ -933,7 +936,6 @@ static int tcp_transmit_skb(struct sock *sk, struct
>>>>sk_buff *skb, int clone_it,
>>>> }
>>>>
>>>> inet = inet_sk(sk);
>>>> - tp = tcp_sk(sk);
>>>> tcb = TCP_SKB_CB(skb);
>>>> memset(&opts, 0, sizeof(opts));
>>>>
>>>> --
>>>> 1.8.1
>>>>
>>
--
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