commit 9bd71be3eef15fa7a037debf9713f7d8db62935f
author Florin Coras <fcoras@cisco.com> Sun Jan 09 18:18:10 2022 -0800
committer Florin Coras <florin.coras@gmail.com> Mon Jan 10 03:12:46 2022 +0000
tree c0581e7c4907b213be82bd0bb0f0d296fc7e6d29
parent 93dd58c8fb020870c770c2a4aa55dcc288fa2035

session: deschedule sessions with no data to send

This ensures the scheduler always tracks sessions that are descheduled,
i.e., do not have events in the old io events list. When app retries to
send, clear descheduled flag and potentially the pacer.

Consequently, transports no longer need to reset the pacer when
sessions are rescheduled after a long app tx pause.

This also fixes a tcp bug whereby the pacer was reset too often when
snd_una was equal to snd_nxt as there was no way to distinguish betwen
app tx breaks and congestion.

Type: improvement

Signed-off-by: Florin Coras <fcoras@cisco.com>
Change-Id: Id3cc6c98cd76299e15030e504380dcf3c04c5189

src/vnet/session/session_node.c

diff --git a/src/vnet/session/session_node.c b/src/vnet/session/session_node.c
index 1d7ebf9..bebabd3 100644
--- a/src/vnet/session/session_node.c
+++ b/src/vnet/session/session_node.c
@@ -1188,8 +1188,14 @@
 
   svm_fifo_unset_event (s->tx_fifo);
   if (svm_fifo_max_dequeue_cons (s->tx_fifo) > ctx->sp.tx_offset)
-    if (svm_fifo_set_event (s->tx_fifo))
-      session_evt_add_head_old (wrk, elt);
+    {
+      if (svm_fifo_set_event (s->tx_fifo))
+	session_evt_add_head_old (wrk, elt);
+    }
+  else
+    {
+      transport_connection_deschedule (ctx->tc);
+    }
 }
 
 always_inline int
@@ -1246,6 +1252,11 @@
 	}
     }
 
+  /* Connection previously descheduled because it had no data to send.
+   * Clear descheduled flag and reset pacer if in use */
+  if (transport_connection_is_descheduled (ctx->tc))
+    transport_connection_clear_descheduled (ctx->tc);
+
   transport_connection_snd_params (ctx->tc, &ctx->sp);
 
   if (!ctx->sp.snd_space)

src/vnet/session/transport.h

diff --git a/src/vnet/session/transport.h b/src/vnet/session/transport.h
index 2cfec06..549f179 100644
--- a/src/vnet/session/transport.h
+++ b/src/vnet/session/transport.h
@@ -327,6 +327,19 @@
   return (tc->flags & TRANSPORT_CONNECTION_F_IS_TX_PACED);
 }
 
+/**
+ * Clear descheduled flag and update pacer if needed
+ *
+ * To add session to scheduler use @ref transport_connection_reschedule
+ */
+always_inline void
+transport_connection_clear_descheduled (transport_connection_t *tc)
+{
+  tc->flags &= ~TRANSPORT_CONNECTION_F_DESCHED;
+  if (transport_connection_is_tx_paced (tc))
+    transport_connection_tx_pacer_reset_bucket (tc, 0 /* bucket */);
+}
+
 u8 *format_transport_pacer (u8 * s, va_list * args);
 
 /**

src/vnet/tcp/tcp_output.c

diff --git a/src/vnet/tcp/tcp_output.c b/src/vnet/tcp/tcp_output.c
index 5647b8f..55fa6d1 100644
--- a/src/vnet/tcp/tcp_output.c
+++ b/src/vnet/tcp/tcp_output.c
@@ -321,7 +321,6 @@
   if (tc->snd_una == tc->snd_nxt)
     {
       tcp_cc_event (tc, TCP_CC_EVT_START_TX);
-      tcp_connection_tx_pacer_reset (tc, tc->cwnd, TRANSPORT_PACER_MIN_BURST);
     }
 
   if (tc->flags & TCP_CONN_PSH_PENDING)