USAGE: Document 'sys_used' metric for 'report footprints'

Signed-off-by: SeongJae Park <sj@kernel.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/USAGE.md b/USAGE.md
index a5e37546..b8a836c8 100644
--- a/USAGE.md
+++ b/USAGE.md
@@ -645,6 +645,8 @@ metrics are supported.
   "virtual set size".
 - `rss`: The amount of physical memory that allocated to the workloads; a.k.a
   "residential set size".
+- `sys_used`: The amount of system's memory that allocated for any usage
+  (`MemTotal - MemFree`).
 
 For example: