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, 217 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-class-net-statistics b/Documentation/ABI/testing/sysfs-class-net-statistics
new file mode 100644
index 000000000..53e508c69
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-class-net-statistics
@@ -0,0 +1,217 @@
+What:		/sys/class/net/<iface>/statistics/collisions
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of collisions seen by this network device.
+		This value might not be relevant with all MAC layers.
+
+What:		/sys/class/net/<iface>/statistics/multicast
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of multicast packets received by this
+		network device.
+
+What:		/sys/class/net/<iface>/statistics/rx_bytes
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of bytes received by this network device.
+		See the network driver for the exact meaning of when this
+		value is incremented.
+
+What:		/sys/class/net/<iface>/statistics/rx_compressed
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of compressed packets received by this
+		network device. This value might only be relevant for interfaces
+		that support packet compression (e.g: PPP).
+
+What:		/sys/class/net/<iface>/statistics/rx_crc_errors
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of packets received with a CRC (FCS) error
+		by this network device. Note that the specific meaning might
+		depend on the MAC layer used by the interface.
+
+What:		/sys/class/net/<iface>/statistics/rx_dropped
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of packets received by the network device
+		but dropped, that are not forwarded to the upper layers for
+		packet processing. See the network driver for the exact
+		meaning of this value.
+
+What:		/sys/class/net/<iface>/statistics/rx_errors
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of receive errors on this network device.
+		See the network driver for the exact meaning of this value.
+
+What:		/sys/class/net/<iface>/statistics/rx_fifo_errors
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of receive FIFO errors seen by this
+		network device. See the network driver for the exact
+		meaning of this value.
+
+What:		/sys/class/net/<iface>/statistics/rx_frame_errors
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of received frames with error, such as
+		alignment errors. Note that the specific meaning depends on
+		on the MAC layer protocol used. See the network driver for
+		the exact meaning of this value.
+
+What:		/sys/class/net/<iface>/statistics/rx_length_errors
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of received error packet with a length
+		error, oversized or undersized. See the network driver for the
+		exact meaning of this value.
+
+What:		/sys/class/net/<iface>/statistics/rx_missed_errors
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of received packets that have been missed
+		due to lack of capacity in the receive side. See the network
+		driver for the exact meaning of this value.
+
+What:		/sys/class/net/<iface>/statistics/rx_nohandler
+Date:		February 2016
+KernelVersion:	4.6
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of received packets that were dropped on
+		an inactive device by the network core.
+
+What:		/sys/class/net/<iface>/statistics/rx_over_errors
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of received packets that are oversized
+		compared to what the network device is configured to accept
+		(e.g: larger than MTU). See the network driver for the exact
+		meaning of this value.
+
+What:		/sys/class/net/<iface>/statistics/rx_packets
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the total number of good packets received by this
+		network device.
+
+What:		/sys/class/net/<iface>/statistics/tx_aborted_errors
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of packets that have been aborted
+		during transmission by a network device (e.g: because of
+		a medium collision). See the network driver for the exact
+		meaning of this value.
+
+What:		/sys/class/net/<iface>/statistics/tx_bytes
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of bytes transmitted by a network
+		device. See the network driver for the exact meaning of this
+		value, in particular whether this accounts for all successfully
+		transmitted packets or all packets that have been queued for
+		transmission.
+
+What:		/sys/class/net/<iface>/statistics/tx_carrier_errors
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of packets that could not be transmitted
+		because of carrier errors (e.g: physical link down). See the
+		network driver for the exact meaning of this value.
+
+What:		/sys/class/net/<iface>/statistics/tx_compressed
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of transmitted compressed packets. Note
+		this might only be relevant for devices that support
+		compression (e.g: PPP).
+
+What:		/sys/class/net/<iface>/statistics/tx_dropped
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of packets dropped during transmission.
+		See the driver for the exact reasons as to why the packets were
+		dropped.
+
+What:		/sys/class/net/<iface>/statistics/tx_errors
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of packets in error during transmission by
+		a network device. See the driver for the exact reasons as to
+		why the packets were dropped.
+
+What:		/sys/class/net/<iface>/statistics/tx_fifo_errors
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of packets having caused a transmit
+		FIFO error. See the driver for the exact reasons as to why the
+		packets were dropped.
+
+What:		/sys/class/net/<iface>/statistics/tx_heartbeat_errors
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of packets transmitted that have been
+		reported as heartbeat errors. See the driver for the exact
+		reasons as to why the packets were dropped.
+
+What:		/sys/class/net/<iface>/statistics/tx_packets
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of packets transmitted by a network
+		device. See the driver for whether this reports the number of all
+		attempted or successful transmissions.
+
+What:		/sys/class/net/<iface>/statistics/tx_window_errors
+Date:		April 2005
+KernelVersion:	2.6.12
+Contact:	netdev@vger.kernel.org
+Description:
+		Indicates the number of packets not successfully transmitted
+		due to a window collision. The specific meaning depends on the
+		MAC layer used.  On Ethernet this is usually used to report
+		late collisions errors.