process-util: Add support SCHED_EXT scheduling policy

Allow CPUSchedulingPolicy to be set to "ext". SCHED_EXT is a new
scheduling policy in Linux v6.12 that allows processes to be scheduled
using custom BPF schedulers instead of the default in-kernel ones.

Selectively setting the SCHED_EXT policy is useful for systems running
in "partial mode" where not all processes are run using a custom
scheduler.

Fallback to SCHED_OTHER and print an error message for systems where
SCHED_EXT isn't available.

src/core/dbus-execute.c

@@ -3167,7 +3167,7 @@ int bus_exec_context_set_transient_property(
                                 return r;
 
                         c->cpu_sched_policy = q;
-                        c->cpu_sched_priority = CLAMP(c->cpu_sched_priority, sched_get_priority_min(q), sched_get_priority_max(q));
+                        c->cpu_sched_priority = CLAMP(c->cpu_sched_priority, sched_get_priority_min_safe(q), sched_get_priority_max_safe(q));
                         c->cpu_sched_set = true;
 
                         unit_write_settingf(u, flags, name, "CPUSchedulingPolicy=%s", s);

src/core/exec-invoke.c

@@ -5473,8 +5473,12 @@ int exec_invoke(
 
                 r = sched_setattr(/* pid= */ 0, &attr, /* flags= */ 0);
                 if (r < 0) {
-                        *exit_status = EXIT_SETSCHEDULER;
-                        return log_error_errno(errno, "Failed to set up CPU scheduling: %m");
+                        if (errno != EINVAL || sched_policy_supported(attr.sched_policy)) {
+                                *exit_status = EXIT_SETSCHEDULER;
+                                return log_error_errno(errno, "Failed to set up CPU scheduling: %m");
+                        }
+
+                        log_warning_errno(errno, "CPU scheduling policy %u is not supported, ignoring: %m", attr.sched_policy);
                 }
         }
 

src/core/load-fragment.c

@@ -1539,9 +1539,12 @@ int config_parse_exec_cpu_sched_policy(
                 return 0;
         }
 
+        if (!sched_policy_supported(x))
+                log_syntax(unit, LOG_WARNING, filename, line, x, "Unsupported CPU scheduling policy: %s", rvalue);
+
         c->cpu_sched_policy = x;
         /* Moving to or from real-time policy? We need to adjust the priority */
-        c->cpu_sched_priority = CLAMP(c->cpu_sched_priority, sched_get_priority_min(x), sched_get_priority_max(x));
+        c->cpu_sched_priority = CLAMP(c->cpu_sched_priority, sched_get_priority_min_safe(x), sched_get_priority_max_safe(x));
         c->cpu_sched_set = true;
 
         return 0;