[PATCH v3 9/9] tcp: Remove broken pressure calculations for tcp_defer_handler()

[David Gibson <david@gibson.dropbear.id.au>]

tcp_defer_handler() performs a potentially expensive linear scan of the
connection table.  So, to mitigate the cost of that we skip if if we're not
under at least moderate pressure: either 30% of available connections or
30% (estimated) of available fds used.

But, the calculation for this has been broken since it was introduced: we
calculate "max_conns" based on c->tcp.conn_count, not TCP_MAX_CONNS,
meaning we only exit early if conn_count is less than 30% of itself, i.e.
never.

If that calculation is "corrected" to be based on TCP_MAX_CONNS, it
completely tanks the TCP CRR times for passt - from ~60ms to >1000ms on my
laptop.  My guess is that this is because in the case of many short lived
connections, we're letting the table become much fuller before compacting
it.  That means that other places which perform a table scan now have to
do much, much more.

For the time being, simply remove the tests, since they're not doing
anything useful.  We can reintroduce them more carefully if we see a need
for them.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 tcp.c | 9 ---------
 1 file changed, 9 deletions(-)

diff --git a/tcp.c b/tcp.c
index f396ede..c89e6e4 100644
--- a/tcp.c
+++ b/tcp.c
@@ -309,9 +309,6 @@
 #define TCP_FRAMES							\
 	(c->mode == MODE_PASST ? TCP_FRAMES_MEM : 1)
 
-#define TCP_FILE_PRESSURE		30	/* % of c->nofile */
-#define TCP_CONN_PRESSURE		30	/* % of c->tcp.conn_count */
-
 #define TCP_HASH_TABLE_LOAD		70		/* % */
 #define TCP_HASH_TABLE_SIZE		(TCP_MAX_CONNS * 100 /		\
 					 TCP_HASH_TABLE_LOAD)
@@ -1385,17 +1382,11 @@ static void tcp_l2_data_buf_flush(struct ctx *c)
  */
 void tcp_defer_handler(struct ctx *c)
 {
-	int max_conns = c->tcp.conn_count / 100 * TCP_CONN_PRESSURE;
-	int max_files = c->nofile / 100 * TCP_FILE_PRESSURE;
 	union tcp_conn *conn;
 
 	tcp_l2_flags_buf_flush(c);
 	tcp_l2_data_buf_flush(c);
 
-	if ((c->tcp.conn_count < MIN(max_files, max_conns)) &&
-	    (c->tcp.splice_conn_count < MIN(max_files / 6, max_conns)))
-		return;
-
 	for (conn = tc + c->tcp.conn_count - 1; conn >= tc; conn--) {
 		if (conn->c.spliced) {
 			if (conn->splice.flags & CLOSING)
-- 
@@ -309,9 +309,6 @@
 #define TCP_FRAMES							\
 	(c->mode == MODE_PASST ? TCP_FRAMES_MEM : 1)
 
-#define TCP_FILE_PRESSURE		30	/* % of c->nofile */
-#define TCP_CONN_PRESSURE		30	/* % of c->tcp.conn_count */
-
 #define TCP_HASH_TABLE_LOAD		70		/* % */
 #define TCP_HASH_TABLE_SIZE		(TCP_MAX_CONNS * 100 /		\
 					 TCP_HASH_TABLE_LOAD)
@@ -1385,17 +1382,11 @@ static void tcp_l2_data_buf_flush(struct ctx *c)
  */
 void tcp_defer_handler(struct ctx *c)
 {
-	int max_conns = c->tcp.conn_count / 100 * TCP_CONN_PRESSURE;
-	int max_files = c->nofile / 100 * TCP_FILE_PRESSURE;
 	union tcp_conn *conn;
 
 	tcp_l2_flags_buf_flush(c);
 	tcp_l2_data_buf_flush(c);
 
-	if ((c->tcp.conn_count < MIN(max_files, max_conns)) &&
-	    (c->tcp.splice_conn_count < MIN(max_files / 6, max_conns)))
-		return;
-
 	for (conn = tc + c->tcp.conn_count - 1; conn >= tc; conn--) {
 		if (conn->c.spliced) {
 			if (conn->splice.flags & CLOSING)
-- 
2.41.0