[PATCH 3/5] sched_ext: Use rcu_dereference_bh() in scx_bypass_lb_timerfn()

[PATCH 3/5] sched_ext: Use rcu_dereference_bh() in scx_bypass_lb_timerfn()

[zhidao su]

From: Su Zhidao <suzhidao@xiaomi.com>

scx_bypass_lb_timerfn() runs in softirq (BH) context, so
rcu_dereference_bh() is the correct RCU accessor. The previous
rcu_dereference_all() suppresses all sparse warnings and masks
potential RCU context issues.

Add a comment noting this is a transitional state: when
multi-scheduler support lands, the bypass LB timer will become
per-scheduler and the global scx_root reference will be removed.

Signed-off-by: Su Zhidao <suzhidao@xiaomi.com>
---
 kernel/sched/ext.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 053d99c58802..c269e489902c 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -4170,7 +4170,13 @@ static void scx_bypass_lb_timerfn(struct timer_list *timer)
 	int node;
 	u32 intv_us;
 
-	sch = rcu_dereference_all(scx_root);
+	/*
+	 * scx_bypass_lb_timer is a global timer that fires in softirq
+	 * context while bypass mode is active. Use rcu_dereference_bh()
+	 * matching the BH context. When multi-scheduler support lands,
+	 * this timer will become per-scheduler instance.
+	 */
+	sch = rcu_dereference_bh(scx_root);
 	if (unlikely(!sch) || !READ_ONCE(scx_bypass_depth))
 		return;
 
-- 
2.43.0

[PATCH 0/5] sched_ext: bypass state machine cleanup and selftest

[zhidao su]

From: Su Zhidao <suzhidao@xiaomi.com>

This series does a small cleanup pass on the sched_ext bypass code path
and adds a selftest for the bypass mechanism.

Patch 1 removes SCX_OPS_HAS_CGROUP_WEIGHT, which was marked deprecated
in 6.15 with a "will be removed on 6.18" comment. We are now past that
point.

Patches 2-3 improve the bypass code in ext.c: add inline comments
explaining the bypass depth counter semantics and the dequeue/enqueue
re-queue loop, and replace rcu_dereference_all() with the more precise
rcu_dereference_bh() in scx_bypass_lb_timerfn() which runs in softirq
context.

Patch 4 adds a selftest that verifies forward progress under bypass
mode: worker processes are spawned while the scheduler is active, then
bpf_link__destroy() is called (triggering bypass), and the test confirms
all workers complete successfully.

Patch 5 adds a comment to the scx_bypass_depth declaration noting its
planned migration into struct scx_sched.

Tested on 6.18.7 with CONFIG_SCHED_CLASS_EXT=y; all existing selftests
pass.

Su Zhidao (5):
  sched_ext: Remove deprecated SCX_OPS_HAS_CGROUP_WEIGHT flag
  sched_ext: Add comments to scx_bypass() for bypass depth semantics
  sched_ext: Use rcu_dereference_bh() in scx_bypass_lb_timerfn()
  sched_ext/selftests: Add bypass mode operational test
  sched_ext: Document scx_bypass_depth migration path

 kernel/sched/ext.c                            |  29 ++++-
 kernel/sched/ext_internal.h                   |   8 +-
 .../sched_ext/include/scx/enum_defs.autogen.h |   1 -
 tools/sched_ext/scx_flatcg.bpf.c              |   2 +-
 tools/testing/selftests/sched_ext/Makefile    |   1 +
 .../testing/selftests/sched_ext/bypass.bpf.c  |  32 ++++++
 tools/testing/selftests/sched_ext/bypass.c    | 105 ++++++++++++++++++
 7 files changed, 165 insertions(+), 13 deletions(-)
 create mode 100644 tools/testing/selftests/sched_ext/bypass.bpf.c
 create mode 100644 tools/testing/selftests/sched_ext/bypass.c

-- 
2.43.0

[PATCH 3/5] sched_ext: Use rcu_dereference_bh() in scx_bypass_lb_timerfn()

[zhidao su]

From: Su Zhidao <suzhidao@xiaomi.com>

scx_bypass_lb_timerfn() runs in softirq (BH) context, so
rcu_dereference_bh() is the correct RCU accessor. The previous
rcu_dereference_all() suppresses all sparse warnings and masks
potential RCU context issues.

Add a comment noting this is a transitional state: when
multi-scheduler support lands, the bypass LB timer will become
per-scheduler and the global scx_root reference will be removed.

Signed-off-by: Su Zhidao <suzhidao@xiaomi.com>
---
 kernel/sched/ext.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 053d99c58802..c269e489902c 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -4170,7 +4170,13 @@ static void scx_bypass_lb_timerfn(struct timer_list *timer)
 	int node;
 	u32 intv_us;
 
-	sch = rcu_dereference_all(scx_root);
+	/*
+	 * scx_bypass_lb_timer is a global timer that fires in softirq
+	 * context while bypass mode is active. Use rcu_dereference_bh()
+	 * matching the BH context. When multi-scheduler support lands,
+	 * this timer will become per-scheduler instance.
+	 */
+	sch = rcu_dereference_bh(scx_root);
 	if (unlikely(!sch) || !READ_ONCE(scx_bypass_depth))
 		return;
 
-- 
2.43.0