[PATCH sched_ext/for-6.18] tools/sched_ext: Add compat helper for scx_bpf_cpu_curr()

[Andrea Righi <arighi@nvidia.com>]

Introduce a compatibility helper that allows BPF schedulers to use
scx_bpf_cpu_curr() on older kernels.

Fixes: 20b158094a1ad ("sched_ext: Introduce scx_bpf_cpu_curr()")
Cc: Christian Loehle <christian.loehle@arm.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 tools/sched_ext/include/scx/common.bpf.h |  2 +-
 tools/sched_ext/include/scx/compat.bpf.h | 17 +++++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/tools/sched_ext/include/scx/common.bpf.h b/tools/sched_ext/include/scx/common.bpf.h
index 342c7c48df5ab..06e2551033cb1 100644
--- a/tools/sched_ext/include/scx/common.bpf.h
+++ b/tools/sched_ext/include/scx/common.bpf.h
@@ -104,7 +104,7 @@ bool scx_bpf_task_running(const struct task_struct *p) __ksym;
 s32 scx_bpf_task_cpu(const struct task_struct *p) __ksym;
 struct rq *scx_bpf_cpu_rq(s32 cpu) __ksym;
 struct rq *scx_bpf_locked_rq(void) __ksym;
-struct task_struct *scx_bpf_cpu_curr(s32 cpu) __ksym;
+struct task_struct *scx_bpf_cpu_curr(s32 cpu) __ksym __weak;
 struct cgroup *scx_bpf_task_cgroup(struct task_struct *p) __ksym __weak;
 u64 scx_bpf_now(void) __ksym __weak;
 void scx_bpf_events(struct scx_event_stats *events, size_t events__sz) __ksym __weak;
diff --git a/tools/sched_ext/include/scx/compat.bpf.h b/tools/sched_ext/include/scx/compat.bpf.h
index 36e0cd2fd4eda..dd9144624dc99 100644
--- a/tools/sched_ext/include/scx/compat.bpf.h
+++ b/tools/sched_ext/include/scx/compat.bpf.h
@@ -230,6 +230,23 @@ static inline bool __COMPAT_is_enq_cpu_selected(u64 enq_flags)
 	 scx_bpf_pick_any_cpu_node(cpus_allowed, node, flags) :			\
 	 scx_bpf_pick_any_cpu(cpus_allowed, flags))
 
+/*
+ * v6.18: Add a helper to retrieve the current task running on a CPU.
+ *
+ * Keep this helper available until v6.20 for compatibility.
+ */
+static inline struct task_struct *__COMPAT_scx_bpf_cpu_curr(int cpu)
+{
+	struct rq *rq;
+
+	if (bpf_ksym_exists(scx_bpf_cpu_curr))
+		return scx_bpf_cpu_curr(cpu);
+
+	rq = scx_bpf_cpu_rq(cpu);
+
+	return rq ? rq->curr : NULL;
+}
+
 /*
  * Define sched_ext_ops. This may be expanded to define multiple variants for
  * backward compatibility. See compat.h::SCX_OPS_LOAD/ATTACH().
-- 
2.51.0