sched_ext: Add scx_bpf_now() for BPF scheduler

scx_bpf_now() is added to the header files so the BPF scheduler
can use it.

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

Author: Changwoo Min

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

@@ -76,6 +76,7 @@ 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 cgroup *scx_bpf_task_cgroup(struct task_struct *p) __ksym __weak;
+u64 scx_bpf_now(void) __ksym __weak;
 
 /*
  * Use the following as @it__iter when calling scx_bpf_dsq_move[_vtime]() from

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

@@ -125,6 +125,11 @@ bool scx_bpf_dispatch_vtime_from_dsq___compat(struct bpf_iter_scx_dsq *it__iter,
 	false;									\
 })
 
+#define scx_bpf_now()								\
+	(bpf_ksym_exists(scx_bpf_now) ?						\
+	 scx_bpf_now() :							\
+	 bpf_ktime_get_ns())
+
 /*
  * Define sched_ext_ops. This may be expanded to define multiple variants for
  * backward compatibility. See compat.h::SCX_OPS_LOAD/ATTACH().