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>

6 files changed, 312 insertions, 0 deletions

diff --git a/Documentation/devicetree/bindings/hwlock/allwinner,sun6i-a31-hwspinlock.yaml b/Documentation/devicetree/bindings/hwlock/allwinner,sun6i-a31-hwspinlock.yaml
new file mode 100644
index 000000000..584cce321
--- /dev/null
+++ b/Documentation/devicetree/bindings/hwlock/allwinner,sun6i-a31-hwspinlock.yaml
@@ -0,0 +1,53 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/hwlock/allwinner,sun6i-a31-hwspinlock.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: SUN6I hardware spinlock driver for Allwinner sun6i compatible SoCs
+
+maintainers:
+  - Wilken Gottwalt <wilken.gottwalt@posteo.net>
+
+description:
+  The hardware unit provides semaphores between the ARM cores and the embedded
+  companion core on the SoC.
+
+properties:
+  compatible:
+    const: allwinner,sun6i-a31-hwspinlock
+
+  reg:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  resets:
+    maxItems: 1
+
+  '#hwlock-cells':
+    const: 1
+
+required:
+  - compatible
+  - reg
+  - clocks
+  - resets
+  - "#hwlock-cells"
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/sun8i-a23-a33-ccu.h>
+    #include <dt-bindings/reset/sun8i-a23-a33-ccu.h>
+
+    hwlock@1c18000 {
+        compatible = "allwinner,sun6i-a31-hwspinlock";
+        reg = <0x01c18000 0x1000>;
+        clocks = <&ccu CLK_BUS_SPINLOCK>;
+        resets = <&ccu RST_BUS_SPINLOCK>;
+        #hwlock-cells = <1>;
+    };
+...
diff --git a/Documentation/devicetree/bindings/hwlock/hwlock.txt b/Documentation/devicetree/bindings/hwlock/hwlock.txt
new file mode 100644
index 000000000..085d1f5c9
--- /dev/null
+++ b/Documentation/devicetree/bindings/hwlock/hwlock.txt
@@ -0,0 +1,59 @@
+Generic hwlock bindings
+=======================
+
+Generic bindings that are common to all the hwlock platform specific driver
+implementations.
+
+Please also look through the individual platform specific hwlock binding
+documentations for identifying any additional properties specific to that
+platform.
+
+hwlock providers:
+=================
+
+Required properties:
+- #hwlock-cells:        Specifies the number of cells needed to represent a
+                        specific lock.
+
+hwlock users:
+=============
+
+Consumers that require specific hwlock(s) should specify them using the
+property "hwlocks", and an optional "hwlock-names" property.
+
+Required properties:
+- hwlocks:              List of phandle to a hwlock provider node and an
+                        associated hwlock args specifier as indicated by
+                        #hwlock-cells. The list can have just a single hwlock
+                        or multiple hwlocks, with each hwlock represented by
+                        a phandle and a corresponding args specifier.
+
+Optional properties:
+- hwlock-names:         List of hwlock name strings defined in the same order
+                        as the hwlocks, with one name per hwlock. Consumers can
+                        use the hwlock-names to match and get a specific hwlock.
+
+
+1. Example of a node using a single specific hwlock:
+
+The following example has a node requesting a hwlock in the bank defined by
+the node hwlock1. hwlock1 is a hwlock provider with an argument specifier
+of length 1.
+
+	node {
+		...
+		hwlocks = <&hwlock1 2>;
+		...
+	};
+
+2. Example of a node using multiple specific hwlocks:
+
+The following example has a node requesting two hwlocks, a hwlock within
+the hwlock device node 'hwlock1' with #hwlock-cells value of 1, and another
+hwlock within the hwlock device node 'hwlock2' with #hwlock-cells value of 2.
+
+	node {
+		...
+		hwlocks = <&hwlock1 2>, <&hwlock2 0 3>;
+		...
+	};
diff --git a/Documentation/devicetree/bindings/hwlock/qcom-hwspinlock.yaml b/Documentation/devicetree/bindings/hwlock/qcom-hwspinlock.yaml
new file mode 100644
index 000000000..ee2726149
--- /dev/null
+++ b/Documentation/devicetree/bindings/hwlock/qcom-hwspinlock.yaml
@@ -0,0 +1,55 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/hwlock/qcom-hwspinlock.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Qualcomm Hardware Mutex Block
+
+maintainers:
+  - Bjorn Andersson <bjorn.andersson@linaro.org>
+
+description:
+  The hardware block provides mutexes utilized between different processors on
+  the SoC as part of the communication protocol used by these processors.
+
+properties:
+  compatible:
+    oneOf:
+      - enum:
+          - qcom,sfpb-mutex
+          - qcom,tcsr-mutex
+      - items:
+          - enum:
+              - qcom,apq8084-tcsr-mutex
+              - qcom,ipq6018-tcsr-mutex
+              - qcom,msm8226-tcsr-mutex
+              - qcom,msm8994-tcsr-mutex
+          - const: qcom,tcsr-mutex
+      - items:
+          - enum:
+              - qcom,msm8974-tcsr-mutex
+          - const: qcom,tcsr-mutex
+          - const: syscon
+
+  reg:
+    maxItems: 1
+
+  '#hwlock-cells':
+    const: 1
+
+required:
+  - compatible
+  - reg
+  - '#hwlock-cells'
+
+additionalProperties: false
+
+examples:
+  - |
+    hwlock@1f40000 {
+        compatible = "qcom,tcsr-mutex";
+        reg = <0x01f40000 0x40000>;
+        #hwlock-cells = <1>;
+    };
+...
diff --git a/Documentation/devicetree/bindings/hwlock/sprd,hwspinlock-r3p0.yaml b/Documentation/devicetree/bindings/hwlock/sprd,hwspinlock-r3p0.yaml
new file mode 100644
index 000000000..abe11df25
--- /dev/null
+++ b/Documentation/devicetree/bindings/hwlock/sprd,hwspinlock-r3p0.yaml
@@ -0,0 +1,50 @@
+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/hwlock/sprd,hwspinlock-r3p0.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Spreadtrum hardware spinlock
+
+maintainers:
+  - Orson Zhai <orsonzhai@gmail.com>
+  - Baolin Wang <baolin.wang7@gmail.com>
+  - Chunyan Zhang <zhang.lyra@gmail.com>
+
+properties:
+  compatible:
+    const: sprd,hwspinlock-r3p0
+
+  reg:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  clock-names:
+    const: enable
+
+  '#hwlock-cells':
+    const: 1
+
+required:
+  - compatible
+  - reg
+  - clocks
+  - clock-names
+  - '#hwlock-cells'
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/sprd,sc9860-clk.h>
+
+    hwlock@40500000 {
+      compatible = "sprd,hwspinlock-r3p0";
+      reg = <0x40500000 0x1000>;
+      clocks = <&aon_gate CLK_SPLK_EB>;
+      clock-names = "enable";
+      #hwlock-cells = <1>;
+    };
+...
diff --git a/Documentation/devicetree/bindings/hwlock/st,stm32-hwspinlock.yaml b/Documentation/devicetree/bindings/hwlock/st,stm32-hwspinlock.yaml
new file mode 100644
index 000000000..829d1fdf4
--- /dev/null
+++ b/Documentation/devicetree/bindings/hwlock/st,stm32-hwspinlock.yaml
@@ -0,0 +1,49 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/hwlock/st,stm32-hwspinlock.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: STMicroelectronics STM32 Hardware Spinlock
+
+maintainers:
+  - Fabien Dessenne <fabien.dessenne@foss.st.com>
+
+properties:
+  "#hwlock-cells":
+    const: 1
+
+  compatible:
+    const: st,stm32-hwspinlock
+
+  reg:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  clock-names:
+    items:
+      - const: hsem
+
+required:
+  - "#hwlock-cells"
+  - compatible
+  - reg
+  - clocks
+  - clock-names
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/stm32mp1-clks.h>
+    hwspinlock@4c000000 {
+        compatible = "st,stm32-hwspinlock";
+        #hwlock-cells = <1>;
+        reg = <0x4c000000 0x400>;
+        clocks = <&rcc HSEM>;
+        clock-names = "hsem";
+    };
+
+...
diff --git a/Documentation/devicetree/bindings/hwlock/ti,omap-hwspinlock.yaml b/Documentation/devicetree/bindings/hwlock/ti,omap-hwspinlock.yaml
new file mode 100644
index 000000000..5ba60d532
--- /dev/null
+++ b/Documentation/devicetree/bindings/hwlock/ti,omap-hwspinlock.yaml
@@ -0,0 +1,46 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/hwlock/ti,omap-hwspinlock.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: TI HwSpinlock for OMAP and K3 based SoCs
+
+maintainers:
+  - Suman Anna <s-anna@ti.com>
+
+properties:
+  compatible:
+    enum:
+      - ti,omap4-hwspinlock  # for OMAP44xx, OMAP54xx, AM33xx, AM43xx, DRA7xx SoCs
+      - ti,am64-hwspinlock   # for K3 AM64x SoCs
+      - ti,am654-hwspinlock  # for K3 AM65x, J721E and J7200 SoCs
+
+  reg:
+    maxItems: 1
+
+  "#hwlock-cells":
+    const: 1
+    description: |
+      The OMAP hwspinlock users will use a 0-indexed relative hwlock number as
+      the argument specifier value for requesting a specific hwspinlock within
+      a hwspinlock bank.
+
+      Please look at the generic hwlock binding for usage information for
+      consumers, "Documentation/devicetree/bindings/hwlock/hwlock.txt"
+
+required:
+  - compatible
+  - reg
+  - "#hwlock-cells"
+
+additionalProperties: false
+
+examples:
+
+  - |
+    spinlock@4a0f6000 {
+        compatible = "ti,omap4-hwspinlock";
+        reg = <0x4a0f6000 0x1000>;
+        #hwlock-cells = <1>;
+    };