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, 292 insertions, 0 deletions

diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
new file mode 100644
index 000000000..1fb6ce684
--- /dev/null
+++ b/drivers/net/loopback.c
@@ -0,0 +1,292 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * INET		An implementation of the TCP/IP protocol suite for the LINUX
+ *		operating system.  INET is implemented using the  BSD Socket
+ *		interface as the means of communication with the user level.
+ *
+ *		Pseudo-driver for the loopback interface.
+ *
+ * Version:	@(#)loopback.c	1.0.4b	08/16/93
+ *
+ * Authors:	Ross Biro
+ *		Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
+ *		Donald Becker, <becker@scyld.com>
+ *
+ *		Alan Cox	:	Fixed oddments for NET3.014
+ *		Alan Cox	:	Rejig for NET3.029 snap #3
+ *		Alan Cox	:	Fixed NET3.029 bugs and sped up
+ *		Larry McVoy	:	Tiny tweak to double performance
+ *		Alan Cox	:	Backed out LMV's tweak - the linux mm
+ *					can't take it...
+ *              Michael Griffith:       Don't bother computing the checksums
+ *                                      on packets received on the loopback
+ *                                      interface.
+ *		Alexey Kuznetsov:	Potential hang under some extreme
+ *					cases removed.
+ */
+#include <linux/kernel.h>
+#include <linux/jiffies.h>
+#include <linux/module.h>
+#include <linux/interrupt.h>
+#include <linux/fs.h>
+#include <linux/types.h>
+#include <linux/string.h>
+#include <linux/socket.h>
+#include <linux/errno.h>
+#include <linux/fcntl.h>
+#include <linux/in.h>
+
+#include <linux/uaccess.h>
+#include <linux/io.h>
+
+#include <linux/inet.h>
+#include <linux/netdevice.h>
+#include <linux/etherdevice.h>
+#include <linux/skbuff.h>
+#include <linux/ethtool.h>
+#include <net/sch_generic.h>
+#include <net/sock.h>
+#include <net/checksum.h>
+#include <linux/if_ether.h>	/* For the statistics structure. */
+#include <linux/if_arp.h>	/* For ARPHRD_ETHER */
+#include <linux/ip.h>
+#include <linux/tcp.h>
+#include <linux/percpu.h>
+#include <linux/net_tstamp.h>
+#include <net/net_namespace.h>
+#include <net/netdev_lock.h>
+#include <linux/u64_stats_sync.h>
+
+/* blackhole_netdev - a device used for dsts that are marked expired!
+ * This is global device (instead of per-net-ns) since it's not needed
+ * to be per-ns and gets initialized at boot time.
+ */
+struct net_device *blackhole_netdev;
+EXPORT_SYMBOL(blackhole_netdev);
+
+/* The higher levels take care of making this non-reentrant (it's
+ * called with bh's disabled).
+ */
+static netdev_tx_t loopback_xmit(struct sk_buff *skb,
+				 struct net_device *dev)
+{
+	int len;
+
+	skb_tx_timestamp(skb);
+
+	/* do not fool net_timestamp_check() with various clock bases */
+	skb_clear_tstamp(skb);
+
+	skb_orphan(skb);
+
+	/* Before queueing this packet to __netif_rx(),
+	 * make sure dst is refcounted.
+	 */
+	skb_dst_force(skb);
+
+	skb->protocol = eth_type_trans(skb, dev);
+
+	len = skb->len;
+	if (likely(__netif_rx(skb) == NET_RX_SUCCESS))
+		dev_lstats_add(dev, len);
+
+	return NETDEV_TX_OK;
+}
+
+void dev_lstats_read(struct net_device *dev, u64 *packets, u64 *bytes)
+{
+	int i;
+
+	*packets = 0;
+	*bytes = 0;
+
+	for_each_possible_cpu(i) {
+		const struct pcpu_lstats *lb_stats;
+		u64 tbytes, tpackets;
+		unsigned int start;
+
+		lb_stats = per_cpu_ptr(dev->lstats, i);
+		do {
+			start = u64_stats_fetch_begin(&lb_stats->syncp);
+			tpackets = u64_stats_read(&lb_stats->packets);
+			tbytes = u64_stats_read(&lb_stats->bytes);
+		} while (u64_stats_fetch_retry(&lb_stats->syncp, start));
+		*bytes   += tbytes;
+		*packets += tpackets;
+	}
+}
+EXPORT_SYMBOL(dev_lstats_read);
+
+static void loopback_get_stats64(struct net_device *dev,
+				 struct rtnl_link_stats64 *stats)
+{
+	u64 packets, bytes;
+
+	dev_lstats_read(dev, &packets, &bytes);
+
+	stats->rx_packets = packets;
+	stats->tx_packets = packets;
+	stats->rx_bytes   = bytes;
+	stats->tx_bytes   = bytes;
+}
+
+static u32 always_on(struct net_device *dev)
+{
+	return 1;
+}
+
+static const struct ethtool_ops loopback_ethtool_ops = {
+	.get_link		= always_on,
+	.get_ts_info		= ethtool_op_get_ts_info,
+};
+
+static int loopback_dev_init(struct net_device *dev)
+{
+	netdev_lockdep_set_classes(dev);
+	return 0;
+}
+
+static void loopback_dev_free(struct net_device *dev)
+{
+	dev_net(dev)->loopback_dev = NULL;
+}
+
+static const struct net_device_ops loopback_ops = {
+	.ndo_init        = loopback_dev_init,
+	.ndo_start_xmit  = loopback_xmit,
+	.ndo_get_stats64 = loopback_get_stats64,
+	.ndo_set_mac_address = eth_mac_addr,
+};
+
+static void gen_lo_setup(struct net_device *dev,
+			 unsigned int mtu,
+			 const struct ethtool_ops *eth_ops,
+			 const struct header_ops *hdr_ops,
+			 const struct net_device_ops *dev_ops,
+			 void (*dev_destructor)(struct net_device *dev))
+{
+	dev->mtu		= mtu;
+	dev->hard_header_len	= ETH_HLEN;	/* 14	*/
+	dev->min_header_len	= ETH_HLEN;	/* 14	*/
+	dev->addr_len		= ETH_ALEN;	/* 6	*/
+	dev->type		= ARPHRD_LOOPBACK;	/* 0x0001*/
+	dev->flags		= IFF_LOOPBACK;
+	dev->priv_flags		|= IFF_LIVE_ADDR_CHANGE | IFF_NO_QUEUE;
+	dev->lltx		= true;
+	dev->netns_immutable	= true;
+	netif_keep_dst(dev);
+	dev->hw_features	= NETIF_F_GSO_SOFTWARE;
+	dev->features		= NETIF_F_SG | NETIF_F_FRAGLIST
+		| NETIF_F_GSO_SOFTWARE
+		| NETIF_F_HW_CSUM
+		| NETIF_F_RXCSUM
+		| NETIF_F_SCTP_CRC
+		| NETIF_F_HIGHDMA
+		| NETIF_F_VLAN_CHALLENGED
+		| NETIF_F_LOOPBACK;
+	dev->ethtool_ops	= eth_ops;
+	dev->header_ops		= hdr_ops;
+	dev->netdev_ops		= dev_ops;
+	dev->needs_free_netdev	= true;
+	dev->pcpu_stat_type	= NETDEV_PCPU_STAT_LSTATS;
+	dev->priv_destructor	= dev_destructor;
+
+	netif_set_tso_max_size(dev, GSO_MAX_SIZE);
+}
+
+/* The loopback device is special. There is only one instance
+ * per network namespace.
+ */
+static void loopback_setup(struct net_device *dev)
+{
+	gen_lo_setup(dev, (64 * 1024), &loopback_ethtool_ops, &eth_header_ops,
+		     &loopback_ops, loopback_dev_free);
+}
+
+/* Setup and register the loopback device. */
+static __net_init int loopback_net_init(struct net *net)
+{
+	struct net_device *dev;
+	int err;
+
+	err = -ENOMEM;
+	dev = alloc_netdev(0, "lo", NET_NAME_PREDICTABLE, loopback_setup);
+	if (!dev)
+		goto out;
+
+	dev_net_set(dev, net);
+	err = register_netdev(dev);
+	if (err)
+		goto out_free_netdev;
+
+	BUG_ON(dev->ifindex != LOOPBACK_IFINDEX);
+	net->loopback_dev = dev;
+	return 0;
+
+out_free_netdev:
+	free_netdev(dev);
+out:
+	if (net_eq(net, &init_net))
+		panic("loopback: Failed to register netdevice: %d\n", err);
+	return err;
+}
+
+/* Registered in net/core/dev.c */
+struct pernet_operations __net_initdata loopback_net_ops = {
+	.init = loopback_net_init,
+};
+
+/* blackhole netdevice */
+static netdev_tx_t blackhole_netdev_xmit(struct sk_buff *skb,
+					 struct net_device *dev)
+{
+	kfree_skb(skb);
+	net_warn_ratelimited("%s(): Dropping skb.\n", __func__);
+	return NETDEV_TX_OK;
+}
+
+static int blackhole_neigh_output(struct neighbour *n, struct sk_buff *skb)
+{
+	kfree_skb(skb);
+	return 0;
+}
+
+static int blackhole_neigh_construct(struct net_device *dev,
+				     struct neighbour *n)
+{
+	n->output = blackhole_neigh_output;
+	return 0;
+}
+
+static const struct net_device_ops blackhole_netdev_ops = {
+	.ndo_start_xmit = blackhole_netdev_xmit,
+	.ndo_neigh_construct = blackhole_neigh_construct,
+};
+
+/* This is a dst-dummy device used specifically for invalidated
+ * DSTs and unlike loopback, this is not per-ns.
+ */
+static void blackhole_netdev_setup(struct net_device *dev)
+{
+	gen_lo_setup(dev, ETH_MIN_MTU, NULL, NULL, &blackhole_netdev_ops, NULL);
+}
+
+/* Setup and register the blackhole_netdev. */
+static int __init blackhole_netdev_init(void)
+{
+	blackhole_netdev = alloc_netdev(0, "blackhole_dev", NET_NAME_UNKNOWN,
+					blackhole_netdev_setup);
+	if (!blackhole_netdev)
+		return -ENOMEM;
+
+	rtnl_net_lock(&init_net);
+	dev_init_scheduler(blackhole_netdev);
+	dev_activate(blackhole_netdev);
+	rtnl_net_unlock(&init_net);
+
+	blackhole_netdev->flags |= IFF_UP | IFF_RUNNING;
+
+	return 0;
+}
+
+device_initcall(blackhole_netdev_init);