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, 119 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-block-device b/Documentation/ABI/testing/sysfs-block-device
new file mode 100644
index 000000000..2d543cfa4
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-block-device
@@ -0,0 +1,119 @@
+What:		/sys/block/*/device/sw_activity
+Date:		Jun, 2008
+KernelVersion:	v2.6.27
+Contact:	linux-ide@vger.kernel.org
+Description:
+		(RW) Used by drivers which support software controlled activity
+		LEDs.
+
+		It has the following valid values:
+
+		==	========================================================
+		0	OFF - the LED is not activated on activity
+		1	BLINK_ON - the LED blinks on every 10ms when activity is
+			detected.
+		2	BLINK_OFF - the LED is on when idle, and blinks off
+			every 10ms when activity is detected.
+		==	========================================================
+
+		Note that the user must turn sw_activity OFF it they wish to
+		control the activity LED via the em_message file.
+
+
+What:		/sys/block/*/device/unload_heads
+Date:		Sep, 2008
+KernelVersion:	v2.6.28
+Contact:	linux-ide@vger.kernel.org
+Description:
+		(RW) Hard disk shock protection
+
+		Writing an integer value to this file will take the heads of the
+		respective drive off the platter and block all I/O operations
+		for the specified number of milliseconds.
+
+		- If the device does not support the unload heads feature,
+		  access is denied with -EOPNOTSUPP.
+		- The maximal value accepted for a timeout is 30000
+		  milliseconds.
+		- A previously set timeout can be cancelled and disk can resume
+		  normal operation immediately by specifying a timeout of 0.
+		- Some hard drives only comply with an earlier version of the
+		  ATA standard, but support the unload feature nonetheless.
+		  There is no safe way Linux can detect these devices, so this
+		  is not enabled by default. If it is known that your device
+		  does support the unload feature, then you can tell the kernel
+		  to enable it by writing -1. It can be disabled again by
+		  writing -2.
+		- Values below -2 are rejected with -EINVAL
+
+		For more information, see
+		Documentation/admin-guide/laptops/disk-shock-protection.rst
+
+
+What:		/sys/block/*/device/ncq_prio_enable
+Date:		Oct, 2016
+KernelVersion:	v4.10
+Contact:	linux-ide@vger.kernel.org
+Description:
+		(RW) Write to the file to turn on or off the SATA NCQ (native
+		command queueing) priority support. By default this feature is
+		turned off. If the device does not support the SATA NCQ
+		priority feature, writing "1" to this file results in an error
+		(see ncq_prio_supported).
+
+
+What:		/sys/block/*/device/sas_ncq_prio_enable
+Date:		Oct, 2016
+KernelVersion:	v4.10
+Contact:	linux-ide@vger.kernel.org
+Description:
+		(RW) This is the equivalent of the ncq_prio_enable attribute
+		file for SATA devices connected to a SAS host-bus-adapter
+		(HBA) implementing support for the SATA NCQ priority feature.
+		This file does not exist if the HBA driver does not implement
+		support for the SATA NCQ priority feature, regardless of the
+		device support for this feature (see sas_ncq_prio_supported).
+
+
+What:		/sys/block/*/device/ncq_prio_supported
+Date:		Aug, 2021
+KernelVersion:	v5.15
+Contact:	linux-ide@vger.kernel.org
+Description:
+		(RO) Indicates if the device supports the SATA NCQ (native
+		command queueing) priority feature.
+
+
+What:		/sys/block/*/device/sas_ncq_prio_supported
+Date:		Aug, 2021
+KernelVersion:	v5.15
+Contact:	linux-ide@vger.kernel.org
+Description:
+		(RO) This is the equivalent of the ncq_prio_supported attribute
+		file for SATA devices connected to a SAS host-bus-adapter
+		(HBA) implementing support for the SATA NCQ priority feature.
+		This file does not exist if the HBA driver does not implement
+		support for the SATA NCQ priority feature, regardless of the
+		device support for this feature.
+
+
+What:		/sys/block/*/device/cdl_supported
+Date:		May, 2023
+KernelVersion:	v6.5
+Contact:	linux-scsi@vger.kernel.org
+Description:
+		(RO) Indicates if the device supports the command duration
+		limits feature found in some ATA and SCSI devices.
+
+
+What:		/sys/block/*/device/cdl_enable
+Date:		May, 2023
+KernelVersion:	v6.5
+Contact:	linux-scsi@vger.kernel.org
+Description:
+		(RW) For a device supporting the command duration limits
+		feature, write to the file to turn on or off the feature.
+		By default this feature is turned off.
+		Writing "1" to this file enables the use of command duration
+		limits for read and write commands in the kernel and turns on
+		the feature on the device. Writing "0" disables the feature.