From a9e3760b0838299649c0d57cca44daaf40ba3c33 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Mon, 14 Sep 2026 22:12:34 -1000
Subject: sched_ext: Maintain an online cid mask in the scheduler arena

Schedulers on the default cid mapping treat [0, nr_online_cids) as the
online set and restart on hotplug. Schedulers that install their own mapping
with scx_bpf_cid_override() have no way to learn which cids are online: the
count no longer identifies members and the CPU-form cpumask is unusable from
cid programs. This is an obvious hole in the cid API.

Add scx_bpf_online_cmask(), a kernel-maintained cmask in the scheduler's
arena, allocated alongside the per-CPU scratch masks and populated after the
cid mapping is finalized and before ops.init(), for child schedulers too.
The pointer stays valid through ops.exit() with no reference to take. It is
the arena offset as a void pointer, the same form struct_ops arena arguments
arrive in. The verifier types the void return as a scalar for the program's
arena cast.

The mask follows the SCX hotplug notifications: seeded from cpu_active_mask
and updated before ops.cid_online/offline() runs, so it lags cpu_online_mask
only inside a hotplug transition. Updates walk the scheduler list under the
lock that also serializes unlinking. Reads are live, not atomic snapshots.
Root initialization excludes hotplug.

v2: Reworded the getter kerneldoc (Andrea Righi).

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
---
 scripts/atomic/fallbacks/acquire             |  5 +++++
 scripts/atomic/fallbacks/add_negative        |  3 +++
 scripts/atomic/fallbacks/add_unless          |  3 +++
 scripts/atomic/fallbacks/andnot              |  3 +++
 scripts/atomic/fallbacks/cmpxchg             |  3 +++
 scripts/atomic/fallbacks/dec                 |  3 +++
 scripts/atomic/fallbacks/dec_and_test        |  3 +++
 scripts/atomic/fallbacks/dec_if_positive     | 11 +++++++++++
 scripts/atomic/fallbacks/dec_unless_positive | 10 ++++++++++
 scripts/atomic/fallbacks/fence               |  7 +++++++
 scripts/atomic/fallbacks/fetch_add_unless    | 10 ++++++++++
 scripts/atomic/fallbacks/inc                 |  3 +++
 scripts/atomic/fallbacks/inc_and_test        |  3 +++
 scripts/atomic/fallbacks/inc_not_zero        |  3 +++
 scripts/atomic/fallbacks/inc_unless_negative | 10 ++++++++++
 scripts/atomic/fallbacks/read_acquire        | 12 ++++++++++++
 scripts/atomic/fallbacks/release             |  4 ++++
 scripts/atomic/fallbacks/set_release         |  8 ++++++++
 scripts/atomic/fallbacks/sub_and_test        |  3 +++
 scripts/atomic/fallbacks/try_cmpxchg         |  7 +++++++
 scripts/atomic/fallbacks/xchg                |  3 +++
 21 files changed, 117 insertions(+)
 create mode 100755 scripts/atomic/fallbacks/acquire
 create mode 100755 scripts/atomic/fallbacks/add_negative
 create mode 100755 scripts/atomic/fallbacks/add_unless
 create mode 100755 scripts/atomic/fallbacks/andnot
 create mode 100644 scripts/atomic/fallbacks/cmpxchg
 create mode 100755 scripts/atomic/fallbacks/dec
 create mode 100755 scripts/atomic/fallbacks/dec_and_test
 create mode 100755 scripts/atomic/fallbacks/dec_if_positive
 create mode 100755 scripts/atomic/fallbacks/dec_unless_positive
 create mode 100755 scripts/atomic/fallbacks/fence
 create mode 100755 scripts/atomic/fallbacks/fetch_add_unless
 create mode 100755 scripts/atomic/fallbacks/inc
 create mode 100755 scripts/atomic/fallbacks/inc_and_test
 create mode 100755 scripts/atomic/fallbacks/inc_not_zero
 create mode 100755 scripts/atomic/fallbacks/inc_unless_negative
 create mode 100755 scripts/atomic/fallbacks/read_acquire
 create mode 100755 scripts/atomic/fallbacks/release
 create mode 100755 scripts/atomic/fallbacks/set_release
 create mode 100755 scripts/atomic/fallbacks/sub_and_test
 create mode 100755 scripts/atomic/fallbacks/try_cmpxchg
 create mode 100644 scripts/atomic/fallbacks/xchg

(limited to 'scripts/atomic/fallbacks')

diff --git a/scripts/atomic/fallbacks/acquire b/scripts/atomic/fallbacks/acquire
new file mode 100755
index 000000000..4da0cab36
--- /dev/null
+++ b/scripts/atomic/fallbacks/acquire
@@ -0,0 +1,5 @@
+cat <<EOF
+	${ret} ret = arch_${atomic}_${pfx}${name}${sfx}_relaxed(${args});
+	__atomic_acquire_fence();
+	return ret;
+EOF
diff --git a/scripts/atomic/fallbacks/add_negative b/scripts/atomic/fallbacks/add_negative
new file mode 100755
index 000000000..1d3d4ab3a
--- /dev/null
+++ b/scripts/atomic/fallbacks/add_negative
@@ -0,0 +1,3 @@
+cat <<EOF
+	return raw_${atomic}_add_return${order}(i, v) < 0;
+EOF
diff --git a/scripts/atomic/fallbacks/add_unless b/scripts/atomic/fallbacks/add_unless
new file mode 100755
index 000000000..95ecb2b74
--- /dev/null
+++ b/scripts/atomic/fallbacks/add_unless
@@ -0,0 +1,3 @@
+cat << EOF
+	return raw_${atomic}_fetch_add_unless(v, a, u) != u;
+EOF
diff --git a/scripts/atomic/fallbacks/andnot b/scripts/atomic/fallbacks/andnot
new file mode 100755
index 000000000..66760457e
--- /dev/null
+++ b/scripts/atomic/fallbacks/andnot
@@ -0,0 +1,3 @@
+cat <<EOF
+	${retstmt}raw_${atomic}_${pfx}and${sfx}${order}(~i, v);
+EOF
diff --git a/scripts/atomic/fallbacks/cmpxchg b/scripts/atomic/fallbacks/cmpxchg
new file mode 100644
index 000000000..1c8507f62
--- /dev/null
+++ b/scripts/atomic/fallbacks/cmpxchg
@@ -0,0 +1,3 @@
+cat <<EOF
+	return raw_cmpxchg${order}(&v->counter, old, new);
+EOF
diff --git a/scripts/atomic/fallbacks/dec b/scripts/atomic/fallbacks/dec
new file mode 100755
index 000000000..60d286d40
--- /dev/null
+++ b/scripts/atomic/fallbacks/dec
@@ -0,0 +1,3 @@
+cat <<EOF
+	${retstmt}raw_${atomic}_${pfx}sub${sfx}${order}(1, v);
+EOF
diff --git a/scripts/atomic/fallbacks/dec_and_test b/scripts/atomic/fallbacks/dec_and_test
new file mode 100755
index 000000000..3a0278e0d
--- /dev/null
+++ b/scripts/atomic/fallbacks/dec_and_test
@@ -0,0 +1,3 @@
+cat <<EOF
+	return raw_${atomic}_dec_return(v) == 0;
+EOF
diff --git a/scripts/atomic/fallbacks/dec_if_positive b/scripts/atomic/fallbacks/dec_if_positive
new file mode 100755
index 000000000..f65c11b4b
--- /dev/null
+++ b/scripts/atomic/fallbacks/dec_if_positive
@@ -0,0 +1,11 @@
+cat <<EOF
+	${int} dec, c = raw_${atomic}_read(v);
+
+	do {
+		dec = c - 1;
+		if (unlikely(dec < 0))
+			break;
+	} while (!raw_${atomic}_try_cmpxchg(v, &c, dec));
+
+	return dec;
+EOF
diff --git a/scripts/atomic/fallbacks/dec_unless_positive b/scripts/atomic/fallbacks/dec_unless_positive
new file mode 100755
index 000000000..d025361d7
--- /dev/null
+++ b/scripts/atomic/fallbacks/dec_unless_positive
@@ -0,0 +1,10 @@
+cat <<EOF
+	${int} c = raw_${atomic}_read(v);
+
+	do {
+		if (unlikely(c > 0))
+			return false;
+	} while (!raw_${atomic}_try_cmpxchg(v, &c, c - 1));
+
+	return true;
+EOF
diff --git a/scripts/atomic/fallbacks/fence b/scripts/atomic/fallbacks/fence
new file mode 100755
index 000000000..40d5b3976
--- /dev/null
+++ b/scripts/atomic/fallbacks/fence
@@ -0,0 +1,7 @@
+cat <<EOF
+	${ret} ret;
+	__atomic_pre_full_fence();
+	ret = arch_${atomic}_${pfx}${name}${sfx}_relaxed(${args});
+	__atomic_post_full_fence();
+	return ret;
+EOF
diff --git a/scripts/atomic/fallbacks/fetch_add_unless b/scripts/atomic/fallbacks/fetch_add_unless
new file mode 100755
index 000000000..8db7e9e17
--- /dev/null
+++ b/scripts/atomic/fallbacks/fetch_add_unless
@@ -0,0 +1,10 @@
+cat << EOF
+	${int} c = raw_${atomic}_read(v);
+
+	do {
+		if (unlikely(c == u))
+			break;
+	} while (!raw_${atomic}_try_cmpxchg(v, &c, c + a));
+
+	return c;
+EOF
diff --git a/scripts/atomic/fallbacks/inc b/scripts/atomic/fallbacks/inc
new file mode 100755
index 000000000..56c770f59
--- /dev/null
+++ b/scripts/atomic/fallbacks/inc
@@ -0,0 +1,3 @@
+cat <<EOF
+	${retstmt}raw_${atomic}_${pfx}add${sfx}${order}(1, v);
+EOF
diff --git a/scripts/atomic/fallbacks/inc_and_test b/scripts/atomic/fallbacks/inc_and_test
new file mode 100755
index 000000000..7d16a10f2
--- /dev/null
+++ b/scripts/atomic/fallbacks/inc_and_test
@@ -0,0 +1,3 @@
+cat <<EOF
+	return raw_${atomic}_inc_return(v) == 0;
+EOF
diff --git a/scripts/atomic/fallbacks/inc_not_zero b/scripts/atomic/fallbacks/inc_not_zero
new file mode 100755
index 000000000..1fcef1e55
--- /dev/null
+++ b/scripts/atomic/fallbacks/inc_not_zero
@@ -0,0 +1,3 @@
+cat <<EOF
+	return raw_${atomic}_add_unless(v, 1, 0);
+EOF
diff --git a/scripts/atomic/fallbacks/inc_unless_negative b/scripts/atomic/fallbacks/inc_unless_negative
new file mode 100755
index 000000000..7b4b09868
--- /dev/null
+++ b/scripts/atomic/fallbacks/inc_unless_negative
@@ -0,0 +1,10 @@
+cat <<EOF
+	${int} c = raw_${atomic}_read(v);
+
+	do {
+		if (unlikely(c < 0))
+			return false;
+	} while (!raw_${atomic}_try_cmpxchg(v, &c, c + 1));
+
+	return true;
+EOF
diff --git a/scripts/atomic/fallbacks/read_acquire b/scripts/atomic/fallbacks/read_acquire
new file mode 100755
index 000000000..e319862d2
--- /dev/null
+++ b/scripts/atomic/fallbacks/read_acquire
@@ -0,0 +1,12 @@
+cat <<EOF
+	${int} ret;
+
+	if (__native_word(${atomic}_t)) {
+		ret = smp_load_acquire(&(v)->counter);
+	} else {
+		ret = raw_${atomic}_read(v);
+		__atomic_acquire_fence();
+	}
+
+	return ret;
+EOF
diff --git a/scripts/atomic/fallbacks/release b/scripts/atomic/fallbacks/release
new file mode 100755
index 000000000..1e6daf57b
--- /dev/null
+++ b/scripts/atomic/fallbacks/release
@@ -0,0 +1,4 @@
+cat <<EOF
+	__atomic_release_fence();
+	${retstmt}arch_${atomic}_${pfx}${name}${sfx}_relaxed(${args});
+EOF
diff --git a/scripts/atomic/fallbacks/set_release b/scripts/atomic/fallbacks/set_release
new file mode 100755
index 000000000..16a374ae6
--- /dev/null
+++ b/scripts/atomic/fallbacks/set_release
@@ -0,0 +1,8 @@
+cat <<EOF
+	if (__native_word(${atomic}_t)) {
+		smp_store_release(&(v)->counter, i);
+	} else {
+		__atomic_release_fence();
+		raw_${atomic}_set(v, i);
+	}
+EOF
diff --git a/scripts/atomic/fallbacks/sub_and_test b/scripts/atomic/fallbacks/sub_and_test
new file mode 100755
index 000000000..d1f746fe0
--- /dev/null
+++ b/scripts/atomic/fallbacks/sub_and_test
@@ -0,0 +1,3 @@
+cat <<EOF
+	return raw_${atomic}_sub_return(i, v) == 0;
+EOF
diff --git a/scripts/atomic/fallbacks/try_cmpxchg b/scripts/atomic/fallbacks/try_cmpxchg
new file mode 100755
index 000000000..d4da82092
--- /dev/null
+++ b/scripts/atomic/fallbacks/try_cmpxchg
@@ -0,0 +1,7 @@
+cat <<EOF
+	${int} r, o = *old;
+	r = raw_${atomic}_cmpxchg${order}(v, o, new);
+	if (unlikely(r != o))
+		*old = r;
+	return likely(r == o);
+EOF
diff --git a/scripts/atomic/fallbacks/xchg b/scripts/atomic/fallbacks/xchg
new file mode 100644
index 000000000..e4def1e0d
--- /dev/null
+++ b/scripts/atomic/fallbacks/xchg
@@ -0,0 +1,3 @@
+cat <<EOF
+	return raw_xchg${order}(&v->counter, new);
+EOF
-- 
cgit v1.3.1