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, 192 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-class-led-trigger-netdev b/Documentation/ABI/testing/sysfs-class-led-trigger-netdev
new file mode 100644
index 000000000..ed46b37ab
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-class-led-trigger-netdev
@@ -0,0 +1,192 @@
+What:		/sys/class/leds/<led>/device_name
+Date:		Dec 2017
+KernelVersion:	4.16
+Contact:	linux-leds@vger.kernel.org
+Description:
+		Specifies the network device name to monitor.
+
+What:		/sys/class/leds/<led>/interval
+Date:		Dec 2017
+KernelVersion:	4.16
+Contact:	linux-leds@vger.kernel.org
+Description:
+		Specifies the duration of the LED blink in milliseconds.
+		Defaults to 50 ms.
+
+		When offloaded is true, the interval value MUST be set to the
+		default value and cannot be changed.
+		Trying to set any value in this specific mode will return
+		an EINVAL error.
+
+What:		/sys/class/leds/<led>/link
+Date:		Dec 2017
+KernelVersion:	4.16
+Contact:	linux-leds@vger.kernel.org
+Description:
+		Signal the link state of the named network device.
+
+		If set to 0 (default), the LED's normal state is off.
+
+		If set to 1, the LED's normal state reflects the link state
+		of the named network device.
+		Setting this value also immediately changes the LED state.
+
+
+What:		/sys/class/leds/<led>/tx
+Date:		Dec 2017
+KernelVersion:	4.16
+Contact:	linux-leds@vger.kernel.org
+Description:
+		Signal transmission of data on the named network device.
+
+		If set to 0 (default), the LED will not blink on transmission.
+
+		If set to 1, the LED will blink for the milliseconds specified
+		in interval to signal transmission.
+
+		When offloaded is true, the blink interval is controlled by
+		hardware and won't reflect the value set in interval.
+
+What:		/sys/class/leds/<led>/rx
+Date:		Dec 2017
+KernelVersion:	4.16
+Contact:	linux-leds@vger.kernel.org
+Description:
+		Signal reception of data on the named network device.
+
+		If set to 0 (default), the LED will not blink on reception.
+
+		If set to 1, the LED will blink for the milliseconds specified
+		in interval to signal reception.
+
+		When offloaded is true, the blink interval is controlled by
+		hardware and won't reflect the value set in interval.
+
+What:		/sys/class/leds/<led>/offloaded
+Date:		Jun 2023
+KernelVersion:	6.5
+Contact:	linux-leds@vger.kernel.org
+Description:
+		Communicate whether the LED trigger modes are offloaded to
+		hardware or whether software fallback is used.
+
+		If 0, the LED is using software fallback to blink.
+
+		If 1, the LED blinking in requested mode is offloaded to
+		hardware.
+
+What:		/sys/class/leds/<led>/link_10
+Date:		Jun 2023
+KernelVersion:	6.5
+Contact:	linux-leds@vger.kernel.org
+Description:
+		Signal the link speed state of 10Mbps of the named network device.
+
+		If set to 0 (default), the LED's normal state is off.
+
+		If set to 1, the LED's normal state reflects the link state
+		speed of 10MBps of the named network device.
+		Setting this value also immediately changes the LED state.
+
+		Present only if the named network device supports 10Mbps link speed.
+
+What:		/sys/class/leds/<led>/link_100
+Date:		Jun 2023
+KernelVersion:	6.5
+Contact:	linux-leds@vger.kernel.org
+Description:
+		Signal the link speed state of 100Mbps of the named network device.
+
+		If set to 0 (default), the LED's normal state is off.
+
+		If set to 1, the LED's normal state reflects the link state
+		speed of 100Mbps of the named network device.
+		Setting this value also immediately changes the LED state.
+
+		Present only if the named network device supports 100Mbps link speed.
+
+What:		/sys/class/leds/<led>/link_1000
+Date:		Jun 2023
+KernelVersion:	6.5
+Contact:	linux-leds@vger.kernel.org
+Description:
+		Signal the link speed state of 1000Mbps of the named network device.
+
+		If set to 0 (default), the LED's normal state is off.
+
+		If set to 1, the LED's normal state reflects the link state
+		speed of 1000Mbps of the named network device.
+		Setting this value also immediately changes the LED state.
+
+		Present only if the named network device supports 1000Mbps link speed.
+
+What:		/sys/class/leds/<led>/link_2500
+Date:		Nov 2023
+KernelVersion:	6.8
+Contact:	linux-leds@vger.kernel.org
+Description:
+		Signal the link speed state of 2500Mbps of the named network device.
+
+		If set to 0 (default), the LED's normal state is off.
+
+		If set to 1, the LED's normal state reflects the link state
+		speed of 2500Mbps of the named network device.
+		Setting this value also immediately changes the LED state.
+
+		Present only if the named network device supports 2500Mbps link speed.
+
+What:		/sys/class/leds/<led>/link_5000
+Date:		Nov 2023
+KernelVersion:	6.8
+Contact:	linux-leds@vger.kernel.org
+Description:
+		Signal the link speed state of 5000Mbps of the named network device.
+
+		If set to 0 (default), the LED's normal state is off.
+
+		If set to 1, the LED's normal state reflects the link state
+		speed of 5000Mbps of the named network device.
+		Setting this value also immediately changes the LED state.
+
+		Present only if the named network device supports 5000Mbps link speed.
+
+What:		/sys/class/leds/<led>/link_10000
+Date:		Nov 2023
+KernelVersion:	6.8
+Contact:	linux-leds@vger.kernel.org
+Description:
+		Signal the link speed state of 10000Mbps of the named network device.
+
+		If set to 0 (default), the LED's normal state is off.
+
+		If set to 1, the LED's normal state reflects the link state
+		speed of 10000Mbps of the named network device.
+		Setting this value also immediately changes the LED state.
+
+		Present only if the named network device supports 10000Mbps link speed.
+
+What:		/sys/class/leds/<led>/half_duplex
+Date:		Jun 2023
+KernelVersion:	6.5
+Contact:	linux-leds@vger.kernel.org
+Description:
+		Signal the link half duplex state of the named network device.
+
+		If set to 0 (default), the LED's normal state is off.
+
+		If set to 1, the LED's normal state reflects the link half
+		duplex state of the named network device.
+		Setting this value also immediately changes the LED state.
+
+What:		/sys/class/leds/<led>/full_duplex
+Date:		Jun 2023
+KernelVersion:	6.5
+Contact:	linux-leds@vger.kernel.org
+Description:
+		Signal the link full duplex state of the named network device.
+
+		If set to 0 (default), the LED's normal state is off.
+
+		If set to 1, the LED's normal state reflects the link full
+		duplex state of the named network device.
+		Setting this value also immediately changes the LED state.