sched_ext: Maintain an online cid mask in the scheduler arenagrafted

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>

1 files changed, 37 insertions, 0 deletions

diff --git a/include/net/nfc/llc.h b/include/net/nfc/llc.h
new file mode 100644
index 000000000..1f699cb17
--- /dev/null
+++ b/include/net/nfc/llc.h
@@ -0,0 +1,37 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Link Layer Control manager public interface
+ *
+ * Copyright (C) 2012  Intel Corporation. All rights reserved.
+ */
+
+#ifndef __NFC_LLC_H_
+#define __NFC_LLC_H_
+
+#include <net/nfc/hci.h>
+#include <linux/skbuff.h>
+
+#define LLC_NOP_NAME "nop"
+#define LLC_SHDLC_NAME "shdlc"
+
+typedef void (*rcv_to_hci_t) (struct nfc_hci_dev *hdev, struct sk_buff *skb);
+typedef int (*xmit_to_drv_t) (struct nfc_hci_dev *hdev, struct sk_buff *skb);
+typedef void (*llc_failure_t) (struct nfc_hci_dev *hdev, int err);
+
+struct nfc_llc;
+
+struct nfc_llc *nfc_llc_allocate(const char *name, struct nfc_hci_dev *hdev,
+				 xmit_to_drv_t xmit_to_drv,
+				 rcv_to_hci_t rcv_to_hci, int tx_headroom,
+				 int tx_tailroom, llc_failure_t llc_failure);
+void nfc_llc_free(struct nfc_llc *llc);
+
+int nfc_llc_start(struct nfc_llc *llc);
+int nfc_llc_stop(struct nfc_llc *llc);
+void nfc_llc_rcv_from_drv(struct nfc_llc *llc, struct sk_buff *skb);
+int nfc_llc_xmit_from_hci(struct nfc_llc *llc, struct sk_buff *skb);
+
+int nfc_llc_init(void);
+void nfc_llc_exit(void);
+
+#endif /* __NFC_LLC_H_ */