bufmon: add buffer monitoring plugin

This plugin allow to keep track of buffer usage in VPP graph nodes. The
main use is to detect buffer leakages.

Type: feature

Change-Id: Iadcf4ab98207fab6e2fa375060879bc2a25b711e
Signed-off-by: Benoît Ganne <bganne@cisco.com>

src/plugins/bufmon/CMakeLists.txt

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+# Copyright (c) 2020 Cisco and/or its affiliates.
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at:
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+add_vpp_plugin(bufmon
+  SOURCES
+  bufmon.c
+)

src/plugins/bufmon/FEATURE.yaml

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+---
+name: Buffers monitoring plugin
+maintainer: Benoît Ganne <bganne@cisco.com>
+features:
+  - monitor buffer utiization in VPP graph nodes
+description: "monitor buffer utiization in VPP graph nodes"
+state: production
+properties: [CLI, MULTITHREAD]

src/plugins/bufmon/bufmon_doc.md

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+# Buffers monitoring plugin {#bufmon_doc}
+
+This plugin enables to track buffer utilization in the VPP graph nodes. The
+main use is to detect buffer leakage.
+It works by keeping track of number of buffer allocations and free in graph
+nodes and also of number of buffers received in input frames and in output
+frames.
+The formula to compute the number of "buffered" buffers in a node is simply:
+        #buffered = #alloc + #input - #free - #output
+Note: monitoring will impact performances.
+
+## Basic usage
+1. Turn buffer traces on:
+```
+~# vppctl set buffer traces on
+```
+2. Monitor buffer usage:
+```
+~# vppctl show buffer traces verbose
+```
+3. Turn buffer traces off:
+```
+~# vppctl set buffer traces off
+```