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, 99 insertions, 0 deletions

diff --git a/Documentation/userspace-api/dcdbas.rst b/Documentation/userspace-api/dcdbas.rst
new file mode 100644
index 000000000..309cc57a7
--- /dev/null
+++ b/Documentation/userspace-api/dcdbas.rst
@@ -0,0 +1,99 @@
+===================================
+Dell Systems Management Base Driver
+===================================
+
+Overview
+========
+
+The Dell Systems Management Base Driver provides a sysfs interface for
+systems management software such as Dell OpenManage to perform system
+management interrupts and host control actions (system power cycle or
+power off after OS shutdown) on certain Dell systems.
+
+Dell OpenManage requires this driver on the following Dell PowerEdge systems:
+300, 1300, 1400, 400SC, 500SC, 1500SC, 1550, 600SC, 1600SC, 650, 1655MC,
+700, and 750.  Other Dell software such as the open source libsmbios project
+is expected to make use of this driver, and it may include the use of this
+driver on other Dell systems.
+
+The Dell libsmbios project aims towards providing access to as much BIOS
+information as possible.  See http://linux.dell.com/libsmbios/main/ for
+more information about the libsmbios project.
+
+
+System Management Interrupt
+===========================
+
+On some Dell systems, systems management software must access certain
+management information via a system management interrupt (SMI).  The SMI data
+buffer must reside in 32-bit address space, and the physical address of the
+buffer is required for the SMI.  The driver maintains the memory required for
+the SMI and provides a way for the application to generate the SMI.
+The driver creates the following sysfs entries for systems management
+software to perform these system management interrupts::
+
+	/sys/devices/platform/dcdbas/smi_data
+	/sys/devices/platform/dcdbas/smi_data_buf_phys_addr
+	/sys/devices/platform/dcdbas/smi_data_buf_size
+	/sys/devices/platform/dcdbas/smi_request
+
+Systems management software must perform the following steps to execute
+a SMI using this driver:
+
+1) Lock smi_data.
+2) Write system management command to smi_data.
+3) Write "1" to smi_request to generate a calling interface SMI or
+   "2" to generate a raw SMI.
+4) Read system management command response from smi_data.
+5) Unlock smi_data.
+
+
+Host Control Action
+===================
+
+Dell OpenManage supports a host control feature that allows the administrator
+to perform a power cycle or power off of the system after the OS has finished
+shutting down.  On some Dell systems, this host control feature requires that
+a driver perform a SMI after the OS has finished shutting down.
+
+The driver creates the following sysfs entries for systems management software
+to schedule the driver to perform a power cycle or power off host control
+action after the system has finished shutting down:
+
+/sys/devices/platform/dcdbas/host_control_action
+/sys/devices/platform/dcdbas/host_control_smi_type
+/sys/devices/platform/dcdbas/host_control_on_shutdown
+
+Dell OpenManage performs the following steps to execute a power cycle or
+power off host control action using this driver:
+
+1) Write host control action to be performed to host_control_action.
+2) Write type of SMI that driver needs to perform to host_control_smi_type.
+3) Write "1" to host_control_on_shutdown to enable host control action.
+4) Initiate OS shutdown.
+   (Driver will perform host control SMI when it is notified that the OS
+   has finished shutting down.)
+
+
+Host Control SMI Type
+=====================
+
+The following table shows the value to write to host_control_smi_type to
+perform a power cycle or power off host control action:
+
+=================== =====================
+PowerEdge System    Host Control SMI Type
+=================== =====================
+      300             HC_SMITYPE_TYPE1
+     1300             HC_SMITYPE_TYPE1
+     1400             HC_SMITYPE_TYPE2
+      500SC           HC_SMITYPE_TYPE2
+     1500SC           HC_SMITYPE_TYPE2
+     1550             HC_SMITYPE_TYPE2
+      600SC           HC_SMITYPE_TYPE2
+     1600SC           HC_SMITYPE_TYPE2
+      650             HC_SMITYPE_TYPE2
+     1655MC           HC_SMITYPE_TYPE2
+      700             HC_SMITYPE_TYPE3
+      750             HC_SMITYPE_TYPE3
+=================== =====================