sched_ext: Make scx_bpf_dsq_insert*() return bool

In preparation for hierarchical schedulers, change scx_bpf_dsq_insert() and
scx_bpf_dsq_insert_vtime() to return bool instead of void. With
sub-schedulers, there will be no reliable way to guarantee a task is still
owned by the sub-scheduler at insertion time (e.g., the task may have been
migrated to another scheduler). The bool return value will enable
sub-schedulers to detect and gracefully handle insertion failures.

For the root scheduler, insertion failures will continue to trigger scheduler
abort via scx_error(), so existing code doesn't need to check the return
value. Backward compatibility is maintained through compat wrappers.

Also update scx_bpf_dsq_move() documentation to clarify that it can return
false for sub-schedulers when @dsq_id points to a disallowed local DSQ.

Reviewed-by: Changwoo Min <changwoo@igalia.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

2 files changed, 22 insertions, 4 deletions

diff --git a/tools/sched_ext/include/scx/common.bpf.h b/tools/sched_ext/include/scx/common.bpf.h
index b1c2a0dde76e..522c90d0ced2 100644
--- a/tools/sched_ext/include/scx/common.bpf.h
+++ b/tools/sched_ext/include/scx/common.bpf.h
@@ -62,8 +62,7 @@ s32 scx_bpf_create_dsq(u64 dsq_id, s32 node) __ksym;
 s32 scx_bpf_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags, bool *is_idle) __ksym;
 s32 __scx_bpf_select_cpu_and(struct task_struct *p, const struct cpumask *cpus_allowed,
 			     struct scx_bpf_select_cpu_and_args *args) __ksym __weak;
-void scx_bpf_dsq_insert(struct task_struct *p, u64 dsq_id, u64 slice, u64 enq_flags) __ksym __weak;
-void __scx_bpf_dsq_insert_vtime(struct task_struct *p, struct scx_bpf_dsq_insert_vtime_args *args) __ksym __weak;
+bool __scx_bpf_dsq_insert_vtime(struct task_struct *p, struct scx_bpf_dsq_insert_vtime_args *args) __ksym __weak;
 u32 scx_bpf_dispatch_nr_slots(void) __ksym;
 void scx_bpf_dispatch_cancel(void) __ksym;
 bool scx_bpf_dsq_move_to_local(u64 dsq_id) __ksym __weak;
diff --git a/tools/sched_ext/include/scx/compat.bpf.h b/tools/sched_ext/include/scx/compat.bpf.h
index e172de696f99..33c26928f4e9 100644
--- a/tools/sched_ext/include/scx/compat.bpf.h
+++ b/tools/sched_ext/include/scx/compat.bpf.h
@@ -196,7 +196,7 @@ scx_bpf_select_cpu_and(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
  * Inline wrapper that packs scalar arguments into a struct and calls
  * __scx_bpf_dsq_insert_vtime(). See __scx_bpf_dsq_insert_vtime() for details.
  */
-static inline void
+static inline bool
 scx_bpf_dsq_insert_vtime(struct task_struct *p, u64 dsq_id, u64 slice, u64 vtime,
 			 u64 enq_flags)
 {
@@ -208,10 +208,29 @@ scx_bpf_dsq_insert_vtime(struct task_struct *p, u64 dsq_id, u64 slice, u64 vtime
 			.enq_flags = enq_flags,
 		};
 
-		__scx_bpf_dsq_insert_vtime(p, &args);
+		return __scx_bpf_dsq_insert_vtime(p, &args);
 	} else {
 		scx_bpf_dsq_insert_vtime___compat(p, dsq_id, slice, vtime,
 						  enq_flags);
+		return true;
+	}
+}
+
+/*
+ * v6.19: scx_bpf_dsq_insert() now returns bool instead of void. Move
+ * scx_bpf_dsq_insert() decl to common.bpf.h and drop compat helper after v6.22.
+ */
+bool scx_bpf_dsq_insert___new(struct task_struct *p, u64 dsq_id, u64 slice, u64 enq_flags) __ksym __weak;
+void scx_bpf_dsq_insert___compat(struct task_struct *p, u64 dsq_id, u64 slice, u64 enq_flags) __ksym __weak;
+
+static inline bool
+scx_bpf_dsq_insert(struct task_struct *p, u64 dsq_id, u64 slice, u64 enq_flags)
+{
+	if (bpf_ksym_exists(scx_bpf_dsq_insert___new)) {
+		return scx_bpf_dsq_insert___new(p, dsq_id, slice, enq_flags);
+	} else {
+		scx_bpf_dsq_insert___compat(p, dsq_id, slice, enq_flags);
+		return true;
 	}
 }