latency-test, latency-histogram: warn when rtapi_app lacks RT privileges

When a run-in-place build is used without 'sudo make setuid' (or
'sudo make setcap'), rtapi_app runs unprivileged: realtime threads get
neither SCHED_FIFO scheduling nor locked memory. The resulting latency
readings are wildly inflated and do not reflect the real hardware, which
is easy to mistake for a code regression.

Add a check in both tools that, for a non-root user, locates rtapi_app
and verifies it is either setuid root or carries the cap_sys_nice file
capability. If neither holds, print a warning that the numbers may be
skewed and point at the missing setuid/setcap step. The check stays
silent when run as root or when rtapi_app cannot be located.

Closes #4044

Author: grandixximo

scripts/latency-histogram

@@ -112,22 +112,17 @@ proc which_exe {name} {
 } ;# which_exe
 
 proc check_rt_privileges {} {
-  # Realtime threads need SCHED_FIFO scheduling and locked memory, which
-  # rtapi_app can only obtain when it is setuid root ('sudo make setuid') or
-  # carries file capabilities ('sudo make setcap').  Without them rtapi_app
-  # runs unprivileged and the measured latency is wildly inflated and
-  # meaningless.  See https://github.com/LinuxCNC/linuxcnc/issues/4044
-  if {![catch {exec id -u} uid] && $uid == 0} return ;# root has all privileges
+  # Without 'sudo make setuid' (or 'sudo make setcap') rtapi_app runs
+  # unprivileged: realtime threads get no SCHED_FIFO and no locked memory, so
+  # the latency readings are wildly inflated.  Warn rather than mislead.
+  if {![catch {exec id -u} uid] && $uid == 0} return ;# root has every privilege
   if {[catch {which_exe rtapi_app} rtapi]} return ;# cannot locate, stay quiet
-  # setuid-root bit ('make setuid')
-  if {![catch {file attributes $rtapi -permissions} mode] && ($mode & 04000)} return
-  # file capabilities ('make setcap')
-  if {![catch {exec getcap $rtapi} caps] && [string match *cap_sys_nice* $caps]} return
+  if {![catch {file attributes $rtapi -permissions} mode] && ($mode & 04000)} return ;# setuid root
+  if {![catch {exec getcap $rtapi} caps] && [string match *cap_sys_nice* $caps]} return ;# file caps
   set msg "Warning: rtapi_app is not setuid root and has no realtime capabilities."
-  append msg "\nRealtime threads cannot get SCHED_FIFO priority or locked memory,"
+  append msg "\nRealtime threads get no SCHED_FIFO priority or locked memory,"
   append msg "\nso the latency shown will be far worse than reality."
-  append msg "\nOn a run-in-place build, run 'sudo make setuid' (or 'sudo make setcap') in src/."
-  append msg "\nSee https://github.com/LinuxCNC/linuxcnc/issues/4044"
+  append msg "\nRun 'sudo make setuid' (or 'sudo make setcap') in src/."
   puts stderr $msg
   if {$::LH(use_x)} {
     package require Tk

scripts/latency-test

@@ -16,29 +16,23 @@ kversion=$(uname -v)
 calc() { awk "BEGIN { print ($1); }" < /dev/null; }
 icalc() { awk "BEGIN { printf \"%.0f\n\", ($1); }" < /dev/null; }
 
-# Realtime threads need SCHED_FIFO scheduling and locked memory, which
-# rtapi_app can only obtain when it is setuid root ('sudo make setuid') or
-# carries file capabilities ('sudo make setcap').  Without them rtapi_app runs
-# unprivileged and the measured latency is wildly inflated and meaningless.
-# See https://github.com/LinuxCNC/linuxcnc/issues/4044
+# Without 'sudo make setuid' (or 'sudo make setcap') rtapi_app runs
+# unprivileged: realtime threads get no SCHED_FIFO and no locked memory, so the
+# latency readings are wildly inflated.  Warn rather than mislead.
 check_rt_privileges () {
-    # root already has every privilege rtapi_app needs
-    [ "$(id -u)" = 0 ] && return 0
+    [ "$(id -u)" = 0 ] && return 0          # root has every privilege
     rtapi_app=$(command -v rtapi_app) || return 0
     [ -n "$rtapi_app" ] || return 0
-    # setuid-root bit ('make setuid')
-    [ -u "$rtapi_app" ] && return 0
-    # file capabilities ('make setcap')
+    [ -u "$rtapi_app" ] && return 0         # setuid root
     if command -v getcap >/dev/null 2>&1; then
         case $(getcap "$rtapi_app" 2>/dev/null) in
-            *cap_sys_nice*) return 0 ;;
+            *cap_sys_nice*) return 0 ;;     # file capabilities
         esac
     fi
     echo "Warning: rtapi_app is not setuid root and has no realtime capabilities." >&2
-    echo "         Realtime threads cannot get SCHED_FIFO priority or locked memory," >&2
-    echo "         so the latency numbers shown below will be far worse than reality." >&2
-    echo "         On a run-in-place build, run 'sudo make setuid' (or 'sudo make setcap')" >&2
-    echo "         in the src/ directory.  See https://github.com/LinuxCNC/linuxcnc/issues/4044" >&2
+    echo "         Realtime threads get no SCHED_FIFO priority or locked memory," >&2
+    echo "         so the latency shown will be far worse than reality." >&2
+    echo "         Run 'sudo make setuid' (or 'sudo make setcap') in src/." >&2
 }
 
 parse_time () {