Re: [PATCH 2/6]: Simplify control flow in the calculation of t_ipi

[Gerrit Renker <gerrit@erg.abdn.ac.uk>]

Quoting Arnaldo Carvalho de Melo:
|  For now I'm merging the patch changing just the commit log not to state that the
|  comment was wrong, i.e. I think its just superfluous :-)
|  
|  As I'm putting these csets in the 'ccid3' branch of my net-2.6.20 git
|  tree if you disagree about any of the things I change today (Sunday)
|  we can always go back and fix it before sending the whole lot to Dave.
If you haven't committed these (they are not in your tree yet), can you please
consider the attached update where I have corrected commit message?
Will put this online, too.

--------------------> Updated Patch <--------------------------------------
[CCID 3]: Simplify control flow in the calculation of t_ipi


This patch performs a simplifying (performance) optimisation:

 In each call of the inline function ccid3_calc_new_t_ipi(), the state is
 tested against TFRC_SSTATE_NO_FBACK. This is expensive when the function
 is called very often. A simpler solution, implemented by this patch, is
 to adapt the control flow.

Background:
-----------
 Upon sender initialisation, the values of t_ipi, t_nom = t_0, delta, as
 well as the initial packet sending rate, are all constants [RFC 3448, 4.2].
 Until feedback arrives, these values are not changed. Hence it is not necessary
 to recalculate t_ipi, t_nom, delta until the first feedback has arrived, i.e.
 as long as the state TFRC_SSTATE_NO_FBACK persists.

Justification:
--------------
 ccid3_calc_new_t_ipi() is called only in two places:
 
   * in ccid3_hc_tx_packet_recv(); here the state is never TFRC_SSTATE_NO_FBACK, due to
	if (hctx->ccid3hctx_state = TFRC_SSTATE_NO_FBACK) {
		ccid3_hc_tx_set_state(sk, TFRC_SSTATE_FBACK);
		/* ... */
   * in ccid3_hc_tx_packet_sent()
	  
 => Only in the second case, the state might be TFRC_SSTATE_NO_FBACK and this only 
     during the initial phase of a connection.

Solution:
---------
 This patch avoids such a recalculation by a simple change of control flow in 
 ccid3_hc_tx_packet_sent(). As a consequence, ccid3_calc_new_t_ipi() is never called in 
 the state TFRC_SSTATE_NO_FBACK, hence the test against this state can safely be removed. 
 Additionally, a comment at the begin of this function is removed.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
---
 net/dccp/ccids/ccid3.c |   10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -103,13 +103,7 @@ static void ccid3_hc_tx_set_state(struct
 /* Calculate new t_ipi (inter packet interval) by t_ipi = s / X_inst */
 static inline void ccid3_calc_new_t_ipi(struct ccid3_hc_tx_sock *hctx)
 {
-	/*
-	 * If no feedback spec says t_ipi is 1 second (set elsewhere and then
-	 * doubles after every no feedback timer (separate function)
-	 */
-	if (hctx->ccid3hctx_state != TFRC_SSTATE_NO_FBACK)
-		hctx->ccid3hctx_t_ipi = usecs_div(hctx->ccid3hctx_s,
-						  hctx->ccid3hctx_x);
+	hctx->ccid3hctx_t_ipi = usecs_div(hctx->ccid3hctx_s, hctx->ccid3hctx_x);
 }
 
 /* Calculate new delta by delta = min(t_ipi / 2, t_gran / 2) */
@@ -395,6 +389,8 @@ static void ccid3_hc_tx_packet_sent(stru
 				  "as a data packet",  dccp_role(sk));
 		return;
 	case TFRC_SSTATE_NO_FBACK:
+		/* t_nom, t_ipi, delta do not change until feedback arrives */
+		return;
 	case TFRC_SSTATE_FBACK:
 		if (len > 0) {
 			timeval_sub_usecs(&hctx->ccid3hctx_t_nom,