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, 100 insertions, 0 deletions

diff --git a/include/net/netns/conntrack.h b/include/net/netns/conntrack.h
new file mode 100644
index 000000000..ab74b5ed0
--- /dev/null
+++ b/include/net/netns/conntrack.h
@@ -0,0 +1,100 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __NETNS_CONNTRACK_H
+#define __NETNS_CONNTRACK_H
+
+#include <linux/list.h>
+#include <linux/list_nulls.h>
+#include <linux/atomic.h>
+#include <linux/workqueue.h>
+#include <linux/netfilter/nf_conntrack_tcp.h>
+#ifdef CONFIG_NF_CT_PROTO_SCTP
+#include <linux/netfilter/nf_conntrack_sctp.h>
+#endif
+#include <linux/seqlock.h>
+
+struct ctl_table_header;
+struct nf_conntrack_ecache;
+
+struct nf_generic_net {
+	unsigned int timeout;
+};
+
+struct nf_tcp_net {
+	unsigned int timeouts[TCP_CONNTRACK_TIMEOUT_MAX];
+	u8 tcp_loose;
+	u8 tcp_be_liberal;
+	u8 tcp_max_retrans;
+	u8 tcp_ignore_invalid_rst;
+#if IS_ENABLED(CONFIG_NF_FLOW_TABLE)
+	unsigned int offload_timeout;
+#endif
+};
+
+enum udp_conntrack {
+	UDP_CT_UNREPLIED,
+	UDP_CT_REPLIED,
+	UDP_CT_MAX
+};
+
+struct nf_udp_net {
+	unsigned int timeouts[UDP_CT_MAX];
+#if IS_ENABLED(CONFIG_NF_FLOW_TABLE)
+	unsigned int offload_timeout;
+#endif
+};
+
+struct nf_icmp_net {
+	unsigned int timeout;
+};
+
+#ifdef CONFIG_NF_CT_PROTO_SCTP
+struct nf_sctp_net {
+	unsigned int timeouts[SCTP_CONNTRACK_MAX];
+};
+#endif
+
+#ifdef CONFIG_NF_CT_PROTO_GRE
+enum gre_conntrack {
+	GRE_CT_UNREPLIED,
+	GRE_CT_REPLIED,
+	GRE_CT_MAX
+};
+
+struct nf_gre_net {
+	struct list_head	keymap_list;
+	unsigned int		timeouts[GRE_CT_MAX];
+};
+#endif
+
+struct nf_ip_net {
+	struct nf_generic_net   generic;
+	struct nf_tcp_net	tcp;
+	struct nf_udp_net	udp;
+	struct nf_icmp_net	icmp;
+	struct nf_icmp_net	icmpv6;
+#ifdef CONFIG_NF_CT_PROTO_SCTP
+	struct nf_sctp_net	sctp;
+#endif
+#ifdef CONFIG_NF_CT_PROTO_GRE
+	struct nf_gre_net	gre;
+#endif
+};
+
+struct netns_ct {
+#ifdef CONFIG_NF_CONNTRACK_EVENTS
+	bool ecache_dwork_pending;
+#endif
+	u8			sysctl_log_invalid; /* Log invalid packets */
+	u8			sysctl_events;
+	u8			sysctl_acct;
+	u8			sysctl_tstamp;
+	u8			sysctl_checksum;
+
+	struct ip_conntrack_stat __percpu *stat;
+	struct nf_ct_event_notifier __rcu *nf_conntrack_event_cb;
+	struct nf_ip_net	nf_ct_proto;
+#if defined(CONFIG_NF_CONNTRACK_LABELS)
+	atomic_t		labels_used;
+#endif
+};
+#endif