dhcrelay: Don't look up the ifindex for the fallback interface (#17797)

Currently, whenever isc-dhcp-relay forwards a packet upstream,
internally, it will try to send it on a "fallback" interface. My
understanding is that this isn't meant to be a real interface, but
instead is basically saying to use Linux's regular routing stack to
route the packet appropriately (rather than having isc-dhcp-relay
specify specifically which interface to use).

The problem is that on systems with a weak CPU, a large number of
interfaces, and many upstream servers specified, this can introduce a
noticeable delay in packets getting sent. The delay comes from trying to
get the ifindex of the fallback interface. In one test case, it got to
the point that only 2 packets could be processed per second. Because of
this, dhcrelay will easily get backlogged and likely get to a point
where packets get dropped in the kernel.

Fix this by adding a check saying if we're using the fallback interface,
then don't try to get the ifindex of this interface. We're never going
to have an interface named this in SONiC.

Signed-off-by: Saikrishna Arcot <sarcot@microsoft.com>

src/isc-dhcp/patch/0016-Don-t-look-up-the-ifindex-for-fallback.patch

@@ -0,0 +1,34 @@
+From 079ff1bb570dae96c4ca513e210c9856e9cc75b0 Mon Sep 17 00:00:00 2001
+From: Saikrishna Arcot <sarcot@microsoft.com>
+Date: Wed, 10 Jan 2024 23:30:17 -0800
+Subject: [PATCH] Don't look up the ifindex for fallback
+
+If sending a packet on the "fallback" interface, then don't try to get the
+ifindex for that interface. There will never be an actual interface named
+"fallback" in SONiC (at least, not one that we will want to use).
+
+This might save 0.009-0.012 seconds per upstream server, and when there
+are as many as 48 upstream servers, it can save about 0.4-0.5 seconds of
+time. This then allows dhcrelay to process more packets.
+
+Signed-off-by: Saikrishna Arcot <sarcot@microsoft.com>
+
+diff --git a/common/socket.c b/common/socket.c
+index da9f501..e707a7f 100644
+--- a/common/socket.c
++++ b/common/socket.c
+@@ -767,7 +767,10 @@ ssize_t send_packet (interface, packet, raw, len, from, to, hto)
+ 	memcpy(&dst, to, sizeof(dst));
+ 	m.msg_name = &dst;
+ 	m.msg_namelen = sizeof(dst);
+-	ifindex = if_nametoindex(interface->name);
++	if (strcmp(interface->name, "fallback") != 0)
++		ifindex = if_nametoindex(interface->name);
++	else
++		ifindex = 0;
+ 
+ 	/*
+ 	 * Set the data buffer we're sending. (Using this wacky
+-- 
+2.34.1
+

src/isc-dhcp/patch/series

@@ -14,3 +14,4 @@
 0013-Fix-dhcrelay-agent-option-buffer-pointer-logic.patch
 0014-enable-parallel-build.patch
 0015-option-to-set-primary-address-in-interface.patch
+0016-Don-t-look-up-the-ifindex-for-fallback.patch