docs: Document priority inversion caveats

Depending on the local system architecture and the game architecture,
changes to the scheduling may result in priority inversion which has
counter-intuitive effects on performance. Let's document these to give
the user a chance to detect and fix this problem.

Signed-off-by: Kai Krakow <kai@kaishome.de>

README.md

@@ -9,6 +9,8 @@ GameMode can leverage support for soft real time mode if the running kernel supp
 
 GameMode adjusts the nice priority of games to -4 by default to give it a slight IO and CPU priority over other background processes. This only works if your user is permitted to adjust priorities within the limits configured by PAM. See `/etc/security/limits.conf`.
 
+Please take note that some games may actually run seemingly slower with `SCHED_ISO` if the game makes use of busy looping while interacting with the graphic driver. The same may happen if you apply too strong nice values. This effect is called priority inversion: Due to the high priority given to busy loops, there may be too few resources left for the graphics driver. Thus, sane defaults were chosen to not expose this effect on most systems. Part of this default is a heuristic which automatically turns off `SCHED_ISO` if GameMode detects three or less CPU cores. Your experience may change based on using GL threaded optimizations, CPU core binding (taskset), the graphic driver, or different CPU architectures. If you experience bad input latency or inconsistent FPS, try switching these configurations on or off first and report back. `SCHED_ISO` comes with a protection against this effect by falling back to normal scheduling as soon as the `SCHED_ISO` process uses more than 70% avarage of all CPU cores. This default value can be adjusted outside of the scope of GameMode (it's in `/proc/sys/kernel/iso_cpu`).
+
 Issues with GameMode should be reported here in the issues section, and not reported to Feral directly.
 
 ---