[services] make snmp.timer work again and delay telemetry.service (#3742)

Delay CPU intensive services at boot

- How I did it
Made snmp.timer work and add telemetry.timer.
But this is not enough because it breaks the existing snmp dependency on swss.
So, in this solution snmp timer is a wanted by swss service, but since OnBootSec timer expires only once it will not trigger snmp service, so I added line "OnUnitActiveSec=0 sec" which will start snmp service based on the last time it was active. On boot only OnBootSec will expire, on swss start/restarts only second timer will expire immediately and trigger snmp service.
However, snmp service will not stop after "systemctl stop snmp" because of the second timer which will always expire when snmp service because unavailable.
So there is a conflict which will be handled by systemd if we add "Conflicts=" line to both snmp.service and snmp.timer.

So during boot:

snmp does not start by default
swss starts and starts snmp timer
OnUnitActiveSec=0 does not expire since there is no snmp active
OnBootSec expires and starts snmp service and snmp timer gets stopped
During "systemctl restart swss"

snmp stops because of Requisite on swss
snmp unblocks snmp timer from running
swss starts and starts snmp timer
OnUnitActiveSec=0 expires imidiately and start snmp which stops snmp timer
During "systemctl stop snmp"

stop of snmp service unblocks snmp timer but no one starts the timer so it is not started by "OnUnitActiveSec=0"

files/build_templates/snmp.service.j2

@@ -4,6 +4,7 @@ Requires=updategraph.service
 Requisite=swss.service
 After=updategraph.service swss.service syncd.service
 Before=ntp-config.service
+Conflicts=snmp.timer
 StartLimitIntervalSec=1200
 StartLimitBurst=3
 
@@ -14,5 +15,3 @@ ExecStop=/usr/bin/{{docker_container_name}}.sh stop
 Restart=always
 RestartSec=30
 
-[Install]
-WantedBy=multi-user.target swss.service

files/build_templates/snmp.timer

@@ -1,9 +1,11 @@
 [Unit]
 Description=Delays snmp container until SONiC has started
+Conflicts=snmp.service
 
 [Timer]
+OnUnitActiveSec=0 sec
 OnBootSec=3min 30 sec
 Unit=snmp.service
 
 [Install]
-WantedBy=timers.target
+WantedBy=timers.target swss.service

files/build_templates/sonic_debian_extension.j2

@@ -386,6 +386,10 @@ sudo LANG=C cp $SCRIPTS_DIR/syncd.sh $FILESYSTEM_ROOT/usr/local/bin/syncd.sh
 # It implements delayed start of services
 sudo cp $BUILD_TEMPLATES/snmp.timer $FILESYSTEM_ROOT/etc/systemd/system/
 sudo LANG=C chroot $FILESYSTEM_ROOT systemctl enable snmp.timer
+{% if enable_system_telemetry == 'y' %}
+sudo cp $BUILD_TEMPLATES/telemetry.timer $FILESYSTEM_ROOT/etc/systemd/system/
+sudo LANG=C chroot $FILESYSTEM_ROOT systemctl enable telemetry.timer
+{% endif %}
 
 sudo LANG=C DEBIAN_FRONTEND=noninteractive chroot $FILESYSTEM_ROOT apt-get purge -y python-dev
 sudo LANG=C DEBIAN_FRONTEND=noninteractive chroot $FILESYSTEM_ROOT apt-get clean -y

files/build_templates/telemetry.service.j2

@@ -14,5 +14,3 @@ ExecStop=/usr/bin/{{docker_container_name}}.sh stop
 Restart=always
 RestartSec=30
 
-[Install]
-WantedBy=multi-user.target

files/build_templates/telemetry.timer

@@ -0,0 +1,9 @@
+[Unit]
+Description=Delays telemetry container until SONiC has started
+
+[Timer]
+OnBootSec=3min 30 sec
+Unit=telemetry.service
+
+[Install]
+WantedBy=timers.target

slave.mk

@@ -639,6 +639,7 @@ $(addprefix $(TARGET_PATH)/, $(SONIC_INSTALLERS)) : $(TARGET_PATH)/% : \
 	export sonic_asic_platform="$(patsubst %-$(CONFIGURED_ARCH),%,$(CONFIGURED_PLATFORM))"
 	export enable_organization_extensions="$(ENABLE_ORGANIZATION_EXTENSIONS)"
 	export enable_dhcp_graph_service="$(ENABLE_DHCP_GRAPH_SERVICE)"
+	export enable_system_telemetry="$(ENABLE_SYSTEM_TELEMETRY)"
 	export shutdown_bgp_on_start="$(SHUTDOWN_BGP_ON_START)"
 	export enable_pfcwd_on_start="$(ENABLE_PFCWD_ON_START)"
 	export installer_debs="$(addprefix $(STRETCH_DEBS_PATH)/,$($*_INSTALLS))"