Support running without tweakable scheduler if cfs profiles are disabled

[ananace]

Currently, if the kernel is built without SCHED_DEBUG, then the entire functionality of the scheduler is prevented - even if the CFS tweaking functionality is never used, especially seeing as cfs-profiles are set as disabled in the default configuration.

I ended up writing a small - and mediocre - patch to be able to get a binary that I can deploy on systems both with and without CFS tuning paths; (Not a rust developer, and not going to submit it as a PR as it should really handle the error properly on startup if cfs-profiles are enabled)

diff --git a/daemon/src/main.rs b/daemon/src/main.rs
index 9823dcb..d9d7bb1 100644
--- a/daemon/src/main.rs
+++ b/daemon/src/main.rs
@@ -158,7 +158,7 @@ async fn daemon(
         return reload(connection).await;
     }
 
-    let service = &mut service::Service::new(owner, SchedPaths::new()?);
+    let service = &mut service::Service::new(owner, SchedPaths::new().ok());
     service.reload_configuration();
 
     let (tx, mut rx) = tokio::sync::mpsc::channel(4);
diff --git a/daemon/src/service.rs b/daemon/src/service.rs
index a252f68..8f0df58 100644
--- a/daemon/src/service.rs
+++ b/daemon/src/service.rs
@@ -12,7 +12,7 @@ use system76_scheduler_config::scheduler::Condition;
 
 pub struct Service<'owner> {
     pub config: crate::config::Config,
-    cfs_paths: SchedPaths,
+    cfs_paths: Option<SchedPaths>,
     foreground_processes: Vec<u32>,
     foreground: Option<u32>,
     pipewire_processes: Vec<u32>,
@@ -21,7 +21,7 @@ pub struct Service<'owner> {
 }
 
 impl<'owner> Service<'owner> {
-    pub fn new(owner: LCellOwner<'owner>, cfs_paths: SchedPaths) -> Self {
+    pub fn new(owner: LCellOwner<'owner>, cfs_paths: Option<SchedPaths>) -> Self {
         Self {
             config: crate::config::Config::default(),
             cfs_paths,
@@ -273,7 +273,7 @@ impl<'owner> Service<'owner> {
             return;
         }
 
-        crate::cfs::tweak(&self.cfs_paths, config);
+        crate::cfs::tweak(&self.cfs_paths.as_ref().unwrap(), config);
     }
 
     pub fn cfs_on_battery(&self, on_battery: bool) {

[mmstick]

The general idea is correct but the unwrap will cause a panic if the debug paths are missing

[ananace]

I was originally considering just doing an if self.cfs_paths.is_some() around the unwrap, but I wanted to keep the patch short and also actually in some way act on the inability to modify the scheduler.
A more correct patch would probably instead only attempt to unwrap the scheduler paths if the configuration file has cfs-profiles enabled - so that you get the correct error out of it. Instead of converting it to an optional and throwing the error out completely.