[PATCH v3] sched_ext: Add tracepoint for scheduler exit

[Pat Somaru]

sched_ext schedulers have state in BPF programs and kernel. scx_dump
provides kernel state and BPF program state on error, but this is static
in what it can provide.

Add a sched_ext_exit tracepoint in scx_claim_exit() so that BPF programs
can dynamically inspect scheduler specific state at the moment of exit.
Pass the exiting scx_sched so attached programs can read its state, and,
since exits propagate through a hierarchy of sub-schedulers, identify
which scheduler each event belongs to.

Signed-off-by: Pat Somaru <patso@xxxxxxxxxxxxxx>
---
Hi Tejun,

> Thanks, but name and level don't pin down a scheduler in a hierarchy -
> same-program instances under sibling cgroups share both. Recording
> sub_cgroup_id and cgrp_path too would make the events self-identifying. Both
> are already on sch. Care to add them in a v3?

Good idea, I added this, thanks!

v3: also record sub_cgroup_id and cgrp_path so events are self-identifying
    in a scheduler hierarchy - name and level don't pin down an instance
    when the same program runs under sibling cgroups (Tejun)
v2: https://lore.kernel.org/all/20260707090314.2567145-1-patso@xxxxxxxxxxxxxx/
v1: https://lore.kernel.org/all/20260512055632.1096713-1-patso@xxxxxxxxxxxxxx/

Tested by running scx_mitosis, attaching to the raw tracepoint and
unregistering the scheduler:

  # bpftrace -e 'rawtracepoint:sched_ext_exit {
        $sch = (struct scx_sched *)arg0;
        printf("sch=%p name=%s level=%d sub_cgroup_id=%llu cgrp_path=%s kind=%ld\n",
               $sch, $sch->ops.name, $sch->level, $sch->ops.sub_cgroup_id,
               str($sch->cgrp_path), (int64)arg1); }'

  Attached 1 probe
  sch=0xff3bd35691464000 name=mitosis_1.1.0_x86_64_unknown_linux_gnu level=0 sub_cgroup_id=0 cgrp_path=/ kind=64

and via the tracepoint's formatted event:

  # grep 'sched_ext_exit:' /sys/kernel/tracing/trace
  scx_mitosis-51 [000] ...1. 30.552740: sched_ext_exit: sched mitosis_1.1.0_x86_64_unknown_linux_gnu level 0 sub_cgroup_id 0 cgrp_path / kind 64

 include/trace/events/sched_ext.h | 28 ++++++++++++++++++++++++++++
 kernel/sched/ext/ext.c           |  2 ++
 kernel/sched/ext/sub.h           |  6 ++++++
 3 files changed, 36 insertions(+)

diff --git a/include/trace/events/sched_ext.h b/include/trace/events/sched_ext.h
index d1bf5acd59c5..1e54f9564ef3 100644
--- a/include/trace/events/sched_ext.h
+++ b/include/trace/events/sched_ext.h
@@ -84,6 +84,34 @@ TRACE_EVENT(sched_ext_bypass_lb,
 	)
 );
 
+TRACE_EVENT(sched_ext_exit,
+
+	TP_PROTO(struct scx_sched *sch, __u32 kind),
+
+	TP_ARGS(sch, kind),
+
+	TP_STRUCT__entry(
+		__string(	name,		sch->ops.name		)
+		__field(	__s32,		level			)
+		__field(	__u64,		sub_cgroup_id		)
+		__string(	cgrp_path,	sch_cgrp_path(sch)	)
+		__field(	__u32,		kind			)
+	),
+
+	TP_fast_assign(
+		__assign_str(name);
+		__entry->level		= sch->level;
+		__entry->sub_cgroup_id	= sch->ops.sub_cgroup_id;
+		__assign_str(cgrp_path);
+		__entry->kind		= kind;
+	),
+
+	TP_printk("sched %s level %d sub_cgroup_id %llu cgrp_path %s kind %u",
+		  __get_str(name), __entry->level, __entry->sub_cgroup_id,
+		  __get_str(cgrp_path), __entry->kind
+	)
+);
+
 #endif /* _TRACE_SCHED_EXT_H */
 
 /* This part must be outside protection */
diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 1a0ec985da77..6ec774435cd1 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -5730,6 +5730,8 @@ static bool scx_claim_exit(struct scx_sched *sch, enum scx_exit_kind kind)
 	 */
 	WRITE_ONCE(sch->aborting, true);
 
+	trace_sched_ext_exit(sch, kind);
+
 	/*
 	 * Propagate exits to descendants immediately. Each has a dedicated
 	 * helper kthread and can run in parallel. While most of disabling is
diff --git a/kernel/sched/ext/sub.h b/kernel/sched/ext/sub.h
index 460a9fd196dc..9b5ac07e5e76 100644
--- a/kernel/sched/ext/sub.h
+++ b/kernel/sched/ext/sub.h
@@ -25,11 +25,17 @@ void scx_sub_disable(struct scx_sched *sch);
 void scx_sub_enable_workfn(struct kthread_work *work);
 bool scx_bpf_sub_dispatch(u64 cgroup_id, const struct bpf_prog_aux *aux);
 
+static inline const char *sch_cgrp_path(struct scx_sched *sch)
+{
+	return sch->cgrp_path;
+}
+
 #else	/* CONFIG_EXT_SUB_SCHED */
 
 static inline struct scx_sched *scx_next_descendant_pre(struct scx_sched *pos, struct scx_sched *root) { return pos ? NULL : root; }
 static inline void scx_set_task_sched(struct task_struct *p, struct scx_sched *sch) {}
 static inline struct cgroup *sch_cgroup(struct scx_sched *sch) { return NULL; }
+static inline const char *sch_cgrp_path(struct scx_sched *sch) { return "/"; }
 static inline void set_cgroup_sched(struct cgroup *cgrp, struct scx_sched *sch) {}
 static inline void drain_descendants(struct scx_sched *sch) { }
 static inline void scx_sub_disable(struct scx_sched *sch) { }

base-commit: 57194a3172ba0123e8f37c4574a8e2863ab67622
-- 
2.55.0