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, 89 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-class-led-flash b/Documentation/ABI/testing/sysfs-class-led-flash
new file mode 100644
index 000000000..11e5677c3
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-class-led-flash
@@ -0,0 +1,89 @@
+What:		/sys/class/leds/<led>/flash_brightness
+Date:		March 2015
+KernelVersion:	4.0
+Contact:	Jacek Anaszewski <j.anaszewski@samsung.com>
+Description:	read/write
+		Set the brightness of this LED in the flash strobe mode, in
+		microamperes. The file is created only for the flash LED devices
+		that support setting flash brightness.
+
+		The value is between 0 and
+		/sys/class/leds/<led>/max_flash_brightness.
+
+What:		/sys/class/leds/<led>/max_flash_brightness
+Date:		March 2015
+KernelVersion:	4.0
+Contact:	Jacek Anaszewski <j.anaszewski@samsung.com>
+Description:	read only
+		Maximum brightness level for this LED in the flash strobe mode,
+		in microamperes.
+
+What:		/sys/class/leds/<led>/flash_timeout
+Date:		March 2015
+KernelVersion:	4.0
+Contact:	Jacek Anaszewski <j.anaszewski@samsung.com>
+Description:	read/write
+		Hardware timeout for flash, in microseconds. The flash strobe
+		is stopped after this period of time has passed from the start
+		of the strobe. The file is created only for the flash LED
+		devices that support setting flash timeout.
+
+What:		/sys/class/leds/<led>/max_flash_timeout
+Date:		March 2015
+KernelVersion:	4.0
+Contact:	Jacek Anaszewski <j.anaszewski@samsung.com>
+Description:	read only
+		Maximum flash timeout for this LED, in microseconds.
+
+What:		/sys/class/leds/<led>/flash_strobe
+Date:		March 2015
+KernelVersion:	4.0
+Contact:	Jacek Anaszewski <j.anaszewski@samsung.com>
+Description:	read/write
+		Flash strobe state. When written with 1 it triggers flash strobe
+		and when written with 0 it turns the flash off.
+
+		On read 1 means that flash is currently strobing and 0 means
+		that flash is off.
+
+What:		/sys/class/leds/<led>/flash_fault
+Date:		March 2015
+KernelVersion:	4.0
+Contact:	Jacek Anaszewski <j.anaszewski@samsung.com>
+Description:	read only
+		Space separated list of flash faults that may have occurred.
+		Flash faults are re-read after strobing the flash. Possible
+		flash faults:
+
+		* led-over-voltage
+			flash controller voltage to the flash LED
+			has exceeded the limit specific to the flash controller
+		* flash-timeout-exceeded
+			the flash strobe was still on when
+			the timeout set by the user has expired; not all flash
+			controllers may set this in all such conditions
+		* controller-over-temperature
+			the flash controller has
+			overheated
+		* controller-short-circuit
+			the short circuit protection
+			of the flash controller has been triggered
+		* led-power-supply-over-current
+			current in the LED power
+			supply has exceeded the limit specific to the flash
+			controller
+		* indicator-led-fault
+			the flash controller has detected
+			a short or open circuit condition on the indicator LED
+		* led-under-voltage
+			flash controller voltage to the flash
+			LED has been below the minimum limit specific to
+			the flash
+		* controller-under-voltage
+			the input voltage of the flash
+			controller is below the limit under which strobing the
+			flash at full current will not be possible;
+			the condition persists until this flag is no longer set
+		* led-over-temperature
+			the temperature of the LED has exceeded
+			its allowed upper limit