From: "Yan, Zheng" <zheng.z.yan@intel.com>
To: "Ilpo Järvinen" <ilpo.jarvinen@helsinki.fi>
Cc: "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
Nandita Dukkipati <nanditad@google.com>
Subject: Re: [PATCH] tcp: properly update lost_cnt_hint during shifting
Date: Wed, 28 Sep 2011 17:15:33 +0800 [thread overview]
Message-ID: <4E82E5B5.1050206@intel.com> (raw)
In-Reply-To: <4E82E0EE.1050600@intel.com>
On 09/28/2011 04:55 PM, Yan, Zheng wrote:
> On 09/28/2011 04:17 PM, Ilpo Järvinen wrote:
>> On Wed, 28 Sep 2011, Yan, Zheng wrote:
>>
>>> lost_skb_hint is used by tcp_mark_head_lost() to mark the first
>>> unhandled skb. lost_cnt_hint is the number of sacked packets before
>>> the lost_skb_hint. tcp_shifted_skb() shouldn't increase lost_cnt_hint
>>> when shifting a sacked skb that is before the lost_skb_hint, because
>>> packets in it are already counted.
>>>
>>> Signed-off-by: Zheng Yan <zheng.z.yan@intel.com>
>>> ---
>>> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
>>> index 21fab3e..f712ace 100644
>>> --- a/net/ipv4/tcp_input.c
>>> +++ b/net/ipv4/tcp_input.c
>>> @@ -1390,9 +1390,14 @@ static int tcp_shifted_skb(struct sock *sk, struct sk_buff *skb,
>>> BUG_ON(!pcount);
>>>
>>> /* Tweak before seqno plays */
>>> - if (!tcp_is_fack(tp) && tcp_is_sack(tp) && tp->lost_skb_hint &&
>>> - !before(TCP_SKB_CB(tp->lost_skb_hint)->seq, TCP_SKB_CB(skb)->seq))
>>> - tp->lost_cnt_hint += pcount;
>>> + if (!tcp_is_fack(tp) && tcp_is_sack(tp) && tp->lost_skb_hint) {
>>> + if (skb == tp->lost_skb_hint)
>>> + tp->lost_cnt_hint += pcount;
>>> + else if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED) &&
>>> + before(TCP_SKB_CB(skb)->seq,
>>> + TCP_SKB_CB(tp->lost_skb_hint)->seq))
>>> + tp->lost_cnt_hint += pcount;
>>> + }
>>
>> Ah right, the hole filled case which shifts not only the newly SACKed
>> skb but also the next, already SACKed skb?
>>
>> I fail to see why you needed to change !before into two checks though:
>> skb == tp->lost_skb_hint and before(params reversed) ? Shouldn't the
>> equality that is provided by the negation cover for the == check (and the
>> params reversion isn't necessary in any case)? In fact, isn't the skb ==
>> tp->lost_skb_hint check strictly wrong without the same TCPCB_SACKED_ACKED
>> guard (though I'm not sure, I didn't check, if the hint can ever point to
>> such a segment in the first place)?
>
> Thanks you for your reply.
>
> skb == tp->lost_skb_hint is special.
>
> If the skb is sacked and we shift 'pcount' packets to previous skb,
> these packets will not be counted by future tcp_mark_head_lost() call.
> So we should increase lost_cnt_hint.
>
> If the skb is not sacked, the skb will be sacked soon by tcp_sacktag_one(),
> So we should not increase lost_cnt_hint.
>
> I didn't think out the second case. I think the correct patch should be:
> ---
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 21fab3e..dcc2411 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -1390,9 +1390,15 @@ static int tcp_shifted_skb(struct sock *sk, struct sk_buff *skb,
> BUG_ON(!pcount);
>
> /* Tweak before seqno plays */
> - if (!tcp_is_fack(tp) && tcp_is_sack(tp) && tp->lost_skb_hint &&
> - !before(TCP_SKB_CB(tp->lost_skb_hint)->seq, TCP_SKB_CB(skb)->seq))
> - tp->lost_cnt_hint += pcount;
> + if (!tcp_is_fack(tp) && tcp_is_sack(tp) && tp->lost_skb_hint) {
> + if ((TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED) &&
> + skb == tp->lost_skb_hint)
> + tp->lost_cnt_hint += pcount;
> + else if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED) &&
> + before(TCP_SKB_CB(skb)->seq,
> + TCP_SKB_CB(tp->lost_skb_hint)->seq))
> + tp->lost_cnt_hint += pcount;
> + }
>
> TCP_SKB_CB(prev)->end_seq += shifted;
> TCP_SKB_CB(skb)->seq += shifted;
> ---
Sorry, I didn't think out the "skb before lost_skb_hint" case neither.
If the skb isn't sacked, tcp_sacktag_one() will increase the lost_cnt_hint.
So tcp_shifted_skb() shouldn't adjust the the lost_cnt_hint.
I hope my patch is correct this time.
---
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 21fab3e..697ce5f 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1390,8 +1390,8 @@ static int tcp_shifted_skb(struct sock *sk, struct sk_buff *skb,
BUG_ON(!pcount);
/* Tweak before seqno plays */
- if (!tcp_is_fack(tp) && tcp_is_sack(tp) && tp->lost_skb_hint &&
- !before(TCP_SKB_CB(tp->lost_skb_hint)->seq, TCP_SKB_CB(skb)->seq))
+ if (!tcp_is_fack(tp) && tcp_is_sack(tp) && tp->lost_skb_hint == skb &&
+ (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
tp->lost_cnt_hint += pcount;
TCP_SKB_CB(prev)->end_seq += shifted;
next prev parent reply other threads:[~2011-09-28 9:15 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-09-28 6:38 [PATCH] tcp: properly update lost_cnt_hint during shifting Yan, Zheng
2011-09-28 8:17 ` Ilpo Järvinen
2011-09-28 8:55 ` Yan, Zheng
2011-09-28 9:15 ` Yan, Zheng [this message]
2011-09-28 9:50 ` Ilpo Järvinen
2011-09-28 10:45 ` Yan, Zheng
2011-09-28 11:29 ` Ilpo Järvinen
2011-09-29 0:06 ` Nandita Dukkipati
2011-09-29 0:12 ` Nandita Dukkipati
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