container: expose priority and SCHED_OTHER policy

The more explicit API removes the arbitrary limit preventing use of SCHED_OTHER (referred to as SCHED_NORMAL in the kernel). This change also exposes priority value to set.

Signed-off-by: Ophestra <cat@gensokyo.uk>

container/container.go

@@ -38,9 +38,13 @@ type (
 	Container struct {
 		// Whether the container init should stay alive after its parent terminates.
 		AllowOrphan bool
-		// Scheduling policy to set via sched_setscheduler(2). The zero value
-		// skips this call. Supported policies are [SCHED_BATCH], [SCHED_IDLE].
+		// Whether to set SchedPolicy and SchedPriority via sched_setscheduler(2).
+		SetScheduler bool
+		// Scheduling policy to set via sched_setscheduler(2).
 		SchedPolicy std.SchedPolicy
+		// Scheduling priority to set via sched_setscheduler(2). The zero value
+		// implies the minimum value supported by the current SchedPolicy.
+		SchedPriority std.Int
 		// Cgroup fd, nil to disable.
 		Cgroup *int
 		// ExtraFiles passed through to initial process in the container, with
@@ -373,7 +377,15 @@ func (p *Container) Start() error {
 
 			// sched_setscheduler: thread-directed but acts on all processes
 			// created from the calling thread
-			if p.SchedPolicy > 0 && p.SchedPolicy <= std.SCHED_LAST {
+			if p.SetScheduler {
+				if p.SchedPolicy < 0 || p.SchedPolicy > std.SCHED_LAST {
+					return &StartError{
+						Fatal: false,
+						Step:  "set scheduling policy",
+						Err:   EINVAL,
+					}
+				}
+
 				var param schedParam
 				if priority, err := p.SchedPolicy.GetPriorityMin(); err != nil {
 					return &StartError{
@@ -382,10 +394,13 @@ func (p *Container) Start() error {
 						Err:   err,
 					}
 				} else {
-					param.priority = priority
+					param.priority = max(priority, p.SchedPriority)
 				}
 
-				p.msg.Verbosef("setting scheduling policy %s", p.SchedPolicy)
+				p.msg.Verbosef(
+					"setting scheduling policy %s priority %d",
+					p.SchedPolicy, param.priority,
+				)
 				if err := schedSetscheduler(
 					0, // calling thread
 					p.SchedPolicy,
@@ -393,7 +408,7 @@ func (p *Container) Start() error {
 				); err != nil {
 					return &StartError{
 						Fatal: true,
-						Step:  "enforce landlock ruleset",
+						Step:  "set scheduling policy",
 						Err:   err,
 					}
 				}

container/std/syscall.go

@@ -134,7 +134,7 @@ func (policy SchedPolicy) GetPriorityMax() (Int, error) {
 			0, 0,
 		)
 		schedPriority[policy][0] = Int(priority)
-		if schedPriority[policy][0] < 0 {
+		if errno != 0 {
 			schedPriorityErr[policy][0] = errno
 		}
 	})
@@ -151,10 +151,9 @@ func (policy SchedPolicy) GetPriorityMin() (Int, error) {
 			0, 0,
 		)
 		schedPriority[policy][1] = Int(priority)
-		if schedPriority[policy][1] < 0 {
+		if errno != 0 {
 			schedPriorityErr[policy][1] = errno
 		}
 	})
 	return schedPriority[policy][1], schedPriorityErr[policy][1]
-
 }

container/syscall.go

@@ -63,12 +63,12 @@ type schedParam struct {
 //
 // [very subtle to use correctly]: https://www.openwall.com/lists/musl/2016/03/01/4
 func schedSetscheduler(tid int, policy std.SchedPolicy, param *schedParam) error {
-	if r, _, errno := Syscall(
+	if _, _, errno := Syscall(
 		SYS_SCHED_SETSCHEDULER,
 		uintptr(tid),
 		uintptr(policy),
 		uintptr(unsafe.Pointer(param)),
-	); r < 0 {
+	); errno != 0 {
 		return errno
 	}
 	return nil