Request SCHED_RR using CAP_SYS_NICE and add Python to nix dev shell

[ry-diffusion]

Describe your PR, what does it fix/add?

• Add missing python3 dep to nix develop
• Change Hyprland process scheduling strategy to SCHED_RR using pthread

Is there anything you want to mention? (unchecked code, possible bugs, found problems, breaking compatibility, etc.)

Also require the maintainers to add SYS_CAP_NICE capability to the package

Is it ready for merging, or does it need work?

I don't think that adding python3 directly into devShell is a good solution, but, works.

[vaxerski]

why

[ry-diffusion]

why

It makes Hyprland more responsive when the computer is under CPU stress, and python is required because libudis86 needs it installed

Example:

$ nix develop
$ meson setup build
# ...
| -- Could NOT find Python3 (missing: Python3_EXECUTABLE Interpreter)
| Building itab.c/itab.h...
| -- Configuring done (1.3s)
| CMake Error at libudis86/CMakeLists.txt:30 (add_library):
| Cannot find source file:
| itab.c

| Tried extensions .c .C .c++ .cc .cpp .cxx .cu .mpp .m .M .mm .ixx .cppm .h
| .hh .h++ .hm .hpp .hxx .in .txx .f .F .for .f77 .f90 .f95 .f03 .hip .ispc


| CMake Error at libudis86/CMakeLists.txt:30 (add_library):
| No SOURCES given to target: libudis86
# ...

---

https://blog.martin-graesslin.com/blog/2017/09/kwinwayland-goes-real-time/
swaywm/sway#6994

[jbeich]

Replacing SCHED_RESET_ON_FORK with pthread_atfork (like Sway) makes it work on FreeBSD with mac_priority(4). For example, cursor in Hyprland no longer decelerates when running poudriere in background.

--- a/src/init/initHelpers.cpp
+++ b/src/init/initHelpers.cpp
@@ -5,11 +5,23 @@ bool Init::isSudo() {
 }
 
 void Init::gainRealTime() {
-#ifdef SCHED_RESET_ON_FORK
     const int   minPrio = sched_get_priority_min(SCHED_RR);
     sched_param param   = {.sched_priority = minPrio};
 
+#ifdef SCHED_RESET_ON_FORK
     if (pthread_setschedparam(pthread_self(), SCHED_RR | SCHED_RESET_ON_FORK, &param))
         Debug::log(WARN, "Failed to change process scheduling strategy");
+#else
+    if (pthread_setschedparam(pthread_self(), SCHED_RR, &param)) {
+        Debug::log(WARN, "Failed to change process scheduling strategy");
+        return;
+    }
+
+    pthread_atfork(NULL, NULL, []() {
+        struct sched_param param = {.sched_priority = 0};
+
+        if (pthread_setschedparam(pthread_self(), SCHED_OTHER, &param))
+            Debug::log(WARN, "Failed to reset process scheduling strategy");
+    });
 #endif
 }
\ No newline at end of file

[ry-diffusion]

if (pthread_setschedparam(pthread_self(), SCHED_RR, &param)) {
+        Debug::log(WARN, "Failed to change process scheduling strategy");
+        return;
+    }

hm, I think that if-guard SCHED_RESET_ON_FORK can be removed, because pthread_atfork method also works on Linux, but SCHED_RESET_ON_FORK looks more integrated with Linux, what do you think?

[jbeich]

Up to @vaxerski. pthread_atfork gives slightly more control, and dropping SCHED_RESET_ON_FORK can reduce maintenance while avoiding build/runtime bugs on non-Linux.

[vaxerski]

I don't care

[vaxerski]

whats the status on this? ready or not

[jbeich]

SCHED_RR part looks OK. I'm not familar with Nix to review.

[vaxerski]

@fufexan

[fufexan]

flake lgtm