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, 152 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-class-powercap b/Documentation/ABI/testing/sysfs-class-powercap
new file mode 100644
index 000000000..ca491ec4e
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-class-powercap
@@ -0,0 +1,152 @@
+What:		/sys/class/powercap/
+Date:		September 2013
+KernelVersion:	3.13
+Contact:	linux-pm@vger.kernel.org
+Description:
+		The powercap/ class sub directory belongs to the power cap
+		subsystem. Refer to
+		Documentation/power/powercap/powercap.rst for details.
+
+What:		/sys/class/powercap/<control type>
+Date:		September 2013
+KernelVersion:	3.13
+Contact:	linux-pm@vger.kernel.org
+Description:
+		A <control type> is a unique name under /sys/class/powercap.
+		Here <control type> determines how the power is going to be
+		controlled. A <control type> can contain multiple power zones.
+
+What:		/sys/class/powercap/<control type>/enabled
+Date:		September 2013
+KernelVersion:	3.13
+Contact:	linux-pm@vger.kernel.org
+Description:
+		This allows to enable/disable power capping for a "control type".
+		This status affects every power zone using this "control_type.
+
+What:		/sys/class/powercap/<control type>/<power zone>
+Date:		September 2013
+KernelVersion:	3.13
+Contact:	linux-pm@vger.kernel.org
+Description:
+		A power zone is a single or a collection of devices, which can
+		be independently monitored and controlled. A power zone sysfs
+		entry is qualified with the name of the <control type>.
+		E.g. intel-rapl:0:1:1.
+
+What:		/sys/class/powercap/<control type>/<power zone>/<child power zone>
+Date:		September 2013
+KernelVersion:	3.13
+Contact:	linux-pm@vger.kernel.org
+Description:
+		Power zones may be organized in a hierarchy in which child
+		power zones provide monitoring and control for a subset of
+		devices under the parent. For example, if there is a parent
+		power zone for a whole CPU package, each CPU core in it can
+		be a child power zone.
+
+What:		/sys/class/powercap/.../<power zone>/name
+Date:		September 2013
+KernelVersion:	3.13
+Contact:	linux-pm@vger.kernel.org
+Description:
+		Specifies the name of this power zone.
+
+What:		/sys/class/powercap/.../<power zone>/energy_uj
+Date:		September 2013
+KernelVersion:	3.13
+Contact:	linux-pm@vger.kernel.org
+Description:
+		Current energy counter in micro-joules. Write "0" to reset.
+		If the counter can not be reset, then this attribute is
+		read-only.
+
+What:		/sys/class/powercap/.../<power zone>/max_energy_range_uj
+Date:		September 2013
+KernelVersion:	3.13
+Contact:	linux-pm@vger.kernel.org
+Description:
+		Range of the above energy counter in micro-joules.
+
+
+What:		/sys/class/powercap/.../<power zone>/power_uw
+Date:		September 2013
+KernelVersion:	3.13
+Contact:	linux-pm@vger.kernel.org
+Description:
+		Current power in micro-watts.
+
+What:		/sys/class/powercap/.../<power zone>/max_power_range_uw
+Date:		September 2013
+KernelVersion:	3.13
+Contact:	linux-pm@vger.kernel.org
+Description:
+		Range of the above power value in micro-watts.
+
+What:		/sys/class/powercap/.../<power zone>/constraint_X_name
+Date:		September 2013
+KernelVersion:	3.13
+Contact:	linux-pm@vger.kernel.org
+Description:
+		Each power zone can define one or more constraints. Each
+		constraint can have an optional name. Here "X" can have values
+		from 0 to max integer.
+
+What:		/sys/class/powercap/.../<power zone>/constraint_X_power_limit_uw
+Date:		September 2013
+KernelVersion:	3.13
+Contact:	linux-pm@vger.kernel.org
+Description:
+		Power limit in micro-watts should be applicable for
+		the time window specified by "constraint_X_time_window_us".
+		Here "X" can have values from 0 to max integer.
+
+What:		/sys/class/powercap/.../<power zone>/constraint_X_time_window_us
+Date:		September 2013
+KernelVersion:	3.13
+Contact:	linux-pm@vger.kernel.org
+Description:
+		Time window in micro seconds. This is used along with
+		constraint_X_power_limit_uw to define a power constraint.
+		Here "X" can have values from 0 to max integer.
+
+
+What:		/sys/class/powercap/<control type>/.../constraint_X_max_power_uw
+Date:		September 2013
+KernelVersion:	3.13
+Contact:	linux-pm@vger.kernel.org
+Description:
+		Maximum allowed power in micro watts for this constraint.
+		Here "X" can have values from 0 to max integer.
+
+What:		/sys/class/powercap/<control type>/.../constraint_X_min_power_uw
+Date:		September 2013
+KernelVersion:	3.13
+Contact:	linux-pm@vger.kernel.org
+Description:
+		Minimum allowed power in micro watts for this constraint.
+		Here "X" can have values from 0 to max integer.
+
+What:		/sys/class/powercap/.../<power zone>/constraint_X_max_time_window_us
+Date:		September 2013
+KernelVersion:	3.13
+Contact:	linux-pm@vger.kernel.org
+Description:
+		Maximum allowed time window in micro seconds for this
+		constraint. Here "X" can have values from 0 to max integer.
+
+What:		/sys/class/powercap/.../<power zone>/constraint_X_min_time_window_us
+Date:		September 2013
+KernelVersion:	3.13
+Contact:	linux-pm@vger.kernel.org
+Description:
+		Minimum allowed time window in micro seconds for this
+		constraint. Here "X" can have values from 0 to max integer.
+
+What:		/sys/class/powercap/.../<power zone>/enabled
+Date:		September 2013
+KernelVersion:	3.13
+Contact:	linux-pm@vger.kernel.org
+Description:
+		This allows to enable/disable power capping at power zone level.
+		This applies to current power zone and its children.