[PATCH][TCP]: simplify tcp_mark_lost_retrans()

[PATCH][TCP]: simplify tcp_mark_lost_retrans()

[Arnd Hannemann]

Hi,

I noticed that in tcp_mark_lost_retrans the for-loop is only entered
if tcp_is_fack(tp) evaluates to true:

           if (!tcp_is_fack(tp) || !tp->retrans_out ||
               !after(received_upto, tp->lost_retrans_low) ||
               icsk->icsk_ca_state != TCP_CA_Recovery)
                   return;

Therefore the following check in the for-loop seems to be redundant,
because it always evaluates to true:

                      (tcp_is_fack(tp) ||
                       !before(received_upto,
                               ack_seq + tp->reordering * tp->mss_cache))

Did I miss something?

Best regards,
Arnd Hannemann

From: Arnd Hannemann <hannemann@nets.rwth-aachen.de>

Because the for loop is only executed for FACK-enabled flows remove
redundant checks within the loop.

Signed-off-by: Arnd Hannemann <hannemann@nets.rwth-aachen.de>
---
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 99b7ecb..cd8b4bd 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1178,10 +1178,7 @@ static void tcp_mark_lost_retrans(struct sock *sk)
    		if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS))
    			continue;

-		if (after(received_upto, ack_seq) &&
-		    (tcp_is_fack(tp) ||
-		     !before(received_upto,
-			     ack_seq + tp->reordering * tp->mss_cache))) {
+		if (after(received_upto, ack_seq)) {
    			TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
    			tp->retrans_out -= tcp_skb_pcount(skb);

Re: [PATCH][TCP]: simplify tcp_mark_lost_retrans()

[Ilpo Järvinen]

On Wed, 7 Jan 2009, Arnd Hannemann wrote:

> I noticed

Good that somebody else is looking TCP code besides me... :-)

> that in tcp_mark_lost_retrans the for-loop is only entered
> if tcp_is_fack(tp) evaluates to true:
> 
>           if (!tcp_is_fack(tp) || !tp->retrans_out ||
>               !after(received_upto, tp->lost_retrans_low) ||
>               icsk->icsk_ca_state != TCP_CA_Recovery)
>                   return;
> 
> Therefore the following check in the for-loop seems to be redundant,
> because it always evaluates to true:
> 
>                      (tcp_is_fack(tp) ||
>                       !before(received_upto,
>                               ack_seq + tp->reordering * tp->mss_cache))
> 
> Did I miss something?

It was just a left over from the RFC3517 SACK addition which added that 
!tcp_is_fack(tp) there above. ...It would have been nice to have similar 
lost rexmit feature without FACK as well but calculating that wasn't 
trivial (or I didn't find that too trivial) and could end up being 
extremely expensive in case of large holes. (So I also left it there as 
sort of reminder).

On the second thought, it would be possible to count skbs we pass while 
walking from the beginning and use that a remaining_sacked counter 
to get rid of all heurestics too and base the counting only on sacked 
stuff which aligns with the spirit of rfc3517 much better than 
sacked+holes used by fack.

> Best regards,
> Arnd Hannemann
> 
> From: Arnd Hannemann <hannemann@nets.rwth-aachen.de>
> 
> Because the for loop is only executed for FACK-enabled flows remove
> redundant checks within the loop.
> 
> Signed-off-by: Arnd Hannemann <hannemann@nets.rwth-aachen.de>
> ---
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 99b7ecb..cd8b4bd 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -1178,10 +1178,7 @@ static void tcp_mark_lost_retrans(struct sock *sk)
>    		if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS))
>    			continue;
> 
> -		if (after(received_upto, ack_seq) &&
> -		    (tcp_is_fack(tp) ||
> -		     !before(received_upto,
> -			     ack_seq + tp->reordering * tp->mss_cache))) {
> +		if (after(received_upto, ack_seq)) {
>    			TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
>    			tp->retrans_out -= tcp_skb_pcount(skb);


-- 
 i.

Re: [PATCH][TCP]: simplify tcp_mark_lost_retrans()

[Ilpo Järvinen]

[-- Attachment #1: Type: TEXT/PLAIN, Size: 1755 bytes --]

On Wed, 7 Jan 2009, Ilpo Järvinen wrote:

> On Wed, 7 Jan 2009, Arnd Hannemann wrote:
> 
> > I noticed
> 
> Good that somebody else is looking TCP code besides me... :-)
> 
> > that in tcp_mark_lost_retrans the for-loop is only entered
> > if tcp_is_fack(tp) evaluates to true:
> > 
> >           if (!tcp_is_fack(tp) || !tp->retrans_out ||
> >               !after(received_upto, tp->lost_retrans_low) ||
> >               icsk->icsk_ca_state != TCP_CA_Recovery)
> >                   return;
> > 
> > Therefore the following check in the for-loop seems to be redundant,
> > because it always evaluates to true:
> > 
> >                      (tcp_is_fack(tp) ||
> >                       !before(received_upto,
> >                               ack_seq + tp->reordering * tp->mss_cache))
> > 
> > Did I miss something?
> 
> It was just a left over from the RFC3517 SACK addition which added that 
> !tcp_is_fack(tp) there above. ...It would have been nice to have similar 
> lost rexmit feature without FACK as well but calculating that wasn't 
> trivial (or I didn't find that too trivial) and could end up being 
> extremely expensive in case of large holes. (So I also left it there as 
> sort of reminder).
> 
> On the second thought, it would be possible to count skbs we pass while 
> walking from the beginning and use that a remaining_sacked counter 
> to get rid of all heurestics too and base the counting only on sacked 
> stuff which aligns with the spirit of rfc3517 much better than 
> sacked+holes used by fack.

Nah, tried to do that that wasn't working nicely either... Since there is 
a need to know how many sack blocks reside between ack_seq and 
received_upto, not the number of sack blocks between skb and 
received_upto...

-- 
 i.

Re: [PATCH][TCP]: simplify tcp_mark_lost_retrans()

[Arnd Hannemann]

Ilpo Järvinen schrieb:
> On Wed, 7 Jan 2009, Arnd Hannemann wrote:
>
>   
>> I noticed
>>     
>
> Good that somebody else is looking TCP code besides me... :-)
>   
Well I try hard... ;-)
>   
>> that in tcp_mark_lost_retrans the for-loop is only entered
>> if tcp_is_fack(tp) evaluates to true:
>>
>>           if (!tcp_is_fack(tp) || !tp->retrans_out ||
>>               !after(received_upto, tp->lost_retrans_low) ||
>>               icsk->icsk_ca_state != TCP_CA_Recovery)
>>                   return;
>>
>> Therefore the following check in the for-loop seems to be redundant,
>> because it always evaluates to true:
>>
>>                      (tcp_is_fack(tp) ||
>>                       !before(received_upto,
>>                               ack_seq + tp->reordering * tp->mss_cache))
>>
>> Did I miss something?
>>     
>
> It was just a left over from the RFC3517 SACK addition which added that 
> !tcp_is_fack(tp) there above. ...It would have been nice to have similar 
> lost rexmit feature without FACK as well but calculating that wasn't 
> trivial (or I didn't find that too trivial) and could end up being 
> extremely expensive in case of large holes. (So I also left it there as 
> sort of reminder).
>   
Perhaps it would be better to let the comments reflect
what you just said and remove the redundant check
anyway to reduce the dead code a newcomer has to understand ;-)
I would have included a patch for the comments, but as you have a
deeper understanding of the code it would probably
be better if you can do it.

Best regards,
Arnd

Re: [PATCH][TCP]: simplify tcp_mark_lost_retrans()

[Ilpo Järvinen]

On Wed, 7 Jan 2009, Arnd Hannemann wrote:

> Perhaps it would be better to let the comments reflect
> what you just said and remove the redundant check
> anyway to reduce the dead code a newcomer has to understand ;-)
> I would have included a patch for the comments, but as you have a
> deeper understanding of the code it would probably
> be better if you can do it.

I have a patch for this in a local tree and will submit it among
other TCP stuff (later on).


-- 
 i.