[PATCH v3 3/3] sched_ext: Expose exit_cpu to BPF and userspace

[Changwoo Min <changwoo@igalia.com>]

Extend struct user_exit_info with an exit_cpu field so BPF schedulers
and the userspace report path can see the CPU that triggered the exit,
matching the kernel-side dump.

UEI_RECORD() defaults the field to -1 before the CO-RE-gated copy so
that running against an older kernel without exit_cpu stays
distinguishable from "exit happened on CPU 0".

UEI_REPORT() appends "on CPU N" to the EXIT line when the value is
valid, surfacing the most diagnostically useful piece of exit info to
any sched_ext userspace tool without needing to crack open the debug
dump.

Signed-off-by: Changwoo Min <changwoo@igalia.com>
---
 tools/sched_ext/include/scx/user_exit_info.bpf.h    | 3 +++
 tools/sched_ext/include/scx/user_exit_info.h        | 2 ++
 tools/sched_ext/include/scx/user_exit_info_common.h | 5 +++++
 3 files changed, 10 insertions(+)

diff --git a/tools/sched_ext/include/scx/user_exit_info.bpf.h b/tools/sched_ext/include/scx/user_exit_info.bpf.h
index e7ac6611a990..98cab643c8d9 100644
--- a/tools/sched_ext/include/scx/user_exit_info.bpf.h
+++ b/tools/sched_ext/include/scx/user_exit_info.bpf.h
@@ -32,6 +32,9 @@
 				  __uei_name##_dump_len, (__ei)->dump);		\
 	if (bpf_core_field_exists((__ei)->exit_code))				\
 		__uei_name.exit_code = (__ei)->exit_code;			\
+	__uei_name.exit_cpu = -1;						\
+	if (bpf_core_field_exists((__ei)->exit_cpu))				\
+		__uei_name.exit_cpu = (__ei)->exit_cpu;				\
 	/* use __sync to force memory barrier */				\
 	__sync_val_compare_and_swap(&__uei_name.kind, __uei_name.kind,		\
 				    (__ei)->kind);				\
diff --git a/tools/sched_ext/include/scx/user_exit_info.h b/tools/sched_ext/include/scx/user_exit_info.h
index 399697fa372f..56a02b549aef 100644
--- a/tools/sched_ext/include/scx/user_exit_info.h
+++ b/tools/sched_ext/include/scx/user_exit_info.h
@@ -39,6 +39,8 @@
 	fprintf(stderr, "EXIT: %s", __uei->reason);				\
 	if (__uei->msg[0] != '\0')						\
 		fprintf(stderr, " (%s)", __uei->msg);				\
+	if (__uei->exit_cpu >= 0)						\
+		fprintf(stderr, " on CPU %d", __uei->exit_cpu);			\
 	fputs("\n", stderr);							\
 	__uei->exit_code;							\
 })
diff --git a/tools/sched_ext/include/scx/user_exit_info_common.h b/tools/sched_ext/include/scx/user_exit_info_common.h
index 2d0981aedd89..76e2a055eb4b 100644
--- a/tools/sched_ext/include/scx/user_exit_info_common.h
+++ b/tools/sched_ext/include/scx/user_exit_info_common.h
@@ -22,6 +22,11 @@ enum uei_sizes {
 
 struct user_exit_info {
 	int		kind;
+	/*
+	 * CPU that triggered the exit, or -1 if unset (e.g. running on an
+	 * older kernel that does not expose this field).
+	 */
+	s32		exit_cpu;
 	s64		exit_code;
 	char		reason[UEI_REASON_LEN];
 	char		msg[UEI_MSG_LEN];
-- 
2.54.0