sched_ext: Add __COMPAT helpers for features added during v6.12 devel cycle

cgroup support and scx_bpf_dispatch[_vtime]_from_dsq() are newly added since
8bb30798 ("sched_ext: Fixes incorrect type in bpf_scx_init()") which is
the current earliest commit targeted by BPF schedulers. Add compat helpers
for them and apply them in the example schedulers.

These will be dropped after a few kernel releases. The exact backward
compatibility window hasn't been decided yet.
Signed-off-by: Tejun Heo <tj@kernel.org>

tools/sched_ext/include/scx/compat.bpf.h

@@ -15,6 +15,25 @@
 	__ret;									\
 })
 
+/* v6.12: 819513666966 ("sched_ext: Add cgroup support") */
+#define __COMPAT_scx_bpf_task_cgroup(p)						\
+	(bpf_ksym_exists(scx_bpf_task_cgroup) ?					\
+	 scx_bpf_task_cgroup((p)) : NULL)
+
+/* v6.12: 4c30f5ce4f7a ("sched_ext: Implement scx_bpf_dispatch[_vtime]_from_dsq()") */
+#define __COMPAT_scx_bpf_dispatch_from_dsq_set_slice(it, slice)			\
+	(bpf_ksym_exists(scx_bpf_dispatch_from_dsq_set_slice) ?			\
+	 scx_bpf_dispatch_from_dsq_set_slice((it), (slice)) : (void)0)
+#define __COMPAT_scx_bpf_dispatch_from_dsq_set_vtime(it, vtime)			\
+	(bpf_ksym_exists(scx_bpf_dispatch_from_dsq_set_vtime) ?			\
+	 scx_bpf_dispatch_from_dsq_set_vtime((it), (vtime)) : (void)0)
+#define __COMPAT_scx_bpf_dispatch_from_dsq(it, p, dsq_id, enq_flags)		\
+	(bpf_ksym_exists(scx_bpf_dispatch_from_dsq) ?				\
+	 scx_bpf_dispatch_from_dsq((it), (p), (dsq_id), (enq_flags)) : false)
+#define __COMPAT_scx_bpf_dispatch_vtime_from_dsq(it, p, dsq_id, enq_flags)	\
+	(bpf_ksym_exists(scx_bpf_dispatch_vtime_from_dsq) ?			\
+	 scx_bpf_dispatch_vtime_from_dsq((it), (p), (dsq_id), (enq_flags)) : false)
+
 /*
  * Define sched_ext_ops. This may be expanded to define multiple variants for
  * backward compatibility. See compat.h::SCX_OPS_LOAD/ATTACH().

tools/sched_ext/scx_flatcg.bpf.c

@@ -383,7 +383,7 @@ void BPF_STRUCT_OPS(fcg_enqueue, struct task_struct *p, u64 enq_flags)
 		return;
 	}
 
-	cgrp = scx_bpf_task_cgroup(p);
+	cgrp = __COMPAT_scx_bpf_task_cgroup(p);
 	cgc = find_cgrp_ctx(cgrp);
 	if (!cgc)
 		goto out_release;
@@ -509,7 +509,7 @@ void BPF_STRUCT_OPS(fcg_runnable, struct task_struct *p, u64 enq_flags)
 {
 	struct cgroup *cgrp;
 
-	cgrp = scx_bpf_task_cgroup(p);
+	cgrp = __COMPAT_scx_bpf_task_cgroup(p);
 	update_active_weight_sums(cgrp, true);
 	bpf_cgroup_release(cgrp);
 }
@@ -522,7 +522,7 @@ void BPF_STRUCT_OPS(fcg_running, struct task_struct *p)
 	if (fifo_sched)
 		return;
 
-	cgrp = scx_bpf_task_cgroup(p);
+	cgrp = __COMPAT_scx_bpf_task_cgroup(p);
 	cgc = find_cgrp_ctx(cgrp);
 	if (cgc) {
 		/*
@@ -565,7 +565,7 @@ void BPF_STRUCT_OPS(fcg_stopping, struct task_struct *p, bool runnable)
 	if (!taskc->bypassed_at)
 		return;
 
-	cgrp = scx_bpf_task_cgroup(p);
+	cgrp = __COMPAT_scx_bpf_task_cgroup(p);
 	cgc = find_cgrp_ctx(cgrp);
 	if (cgc) {
 		__sync_fetch_and_add(&cgc->cvtime_delta,
@@ -579,7 +579,7 @@ void BPF_STRUCT_OPS(fcg_quiescent, struct task_struct *p, u64 deq_flags)
 {
 	struct cgroup *cgrp;
 
-	cgrp = scx_bpf_task_cgroup(p);
+	cgrp = __COMPAT_scx_bpf_task_cgroup(p);
 	update_active_weight_sums(cgrp, false);
 	bpf_cgroup_release(cgrp);
 }

tools/sched_ext/scx_qmap.bpf.c

@@ -318,11 +318,11 @@ static bool dispatch_highpri(bool from_timer)
 
 		if (tctx->highpri) {
 			/* exercise the set_*() and vtime interface too */
-			scx_bpf_dispatch_from_dsq_set_slice(
+			__COMPAT_scx_bpf_dispatch_from_dsq_set_slice(
 				BPF_FOR_EACH_ITER, slice_ns * 2);
-			scx_bpf_dispatch_from_dsq_set_vtime(
+			__COMPAT_scx_bpf_dispatch_from_dsq_set_vtime(
 				BPF_FOR_EACH_ITER, highpri_seq++);
-			scx_bpf_dispatch_vtime_from_dsq(
+			__COMPAT_scx_bpf_dispatch_vtime_from_dsq(
 				BPF_FOR_EACH_ITER, p, HIGHPRI_DSQ, 0);
 		}
 	}
@@ -340,7 +340,7 @@ static bool dispatch_highpri(bool from_timer)
 		else
 			cpu = scx_bpf_pick_any_cpu(p->cpus_ptr, 0);
 
-		if (scx_bpf_dispatch_from_dsq(BPF_FOR_EACH_ITER, p,
+		if (__COMPAT_scx_bpf_dispatch_from_dsq(BPF_FOR_EACH_ITER, p,
 						       SCX_DSQ_LOCAL_ON | cpu,
 						       SCX_ENQ_PREEMPT)) {
 			if (cpu == this_cpu) {