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, 140 insertions, 0 deletions

diff --git a/scripts/gcc-plugins/gcc-generate-simple_ipa-pass.h b/scripts/gcc-plugins/gcc-generate-simple_ipa-pass.h
new file mode 100644
index 000000000..33093ccc9
--- /dev/null
+++ b/scripts/gcc-plugins/gcc-generate-simple_ipa-pass.h
@@ -0,0 +1,140 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Generator for SIMPLE_IPA pass related boilerplate code/data
+ *
+ * Supports gcc 4.5-6
+ *
+ * Usage:
+ *
+ * 1. before inclusion define PASS_NAME
+ * 2. before inclusion define NO_* for unimplemented callbacks
+ *    NO_GATE
+ *    NO_EXECUTE
+ * 3. before inclusion define PROPERTIES_* and TODO_FLAGS_* to override
+ *    the default 0 values
+ * 4. for convenience, all the above will be undefined after inclusion!
+ * 5. the only exported name is make_PASS_NAME_pass() to register with gcc
+ */
+
+#ifndef PASS_NAME
+#error at least PASS_NAME must be defined
+#else
+#define __GCC_PLUGIN_STRINGIFY(n)	#n
+#define _GCC_PLUGIN_STRINGIFY(n)	__GCC_PLUGIN_STRINGIFY(n)
+#define _GCC_PLUGIN_CONCAT2(x, y)	x ## y
+#define _GCC_PLUGIN_CONCAT3(x, y, z)	x ## y ## z
+
+#define __PASS_NAME_PASS_DATA(n)	_GCC_PLUGIN_CONCAT2(n, _pass_data)
+#define _PASS_NAME_PASS_DATA		__PASS_NAME_PASS_DATA(PASS_NAME)
+
+#define __PASS_NAME_PASS(n)		_GCC_PLUGIN_CONCAT2(n, _pass)
+#define _PASS_NAME_PASS			__PASS_NAME_PASS(PASS_NAME)
+
+#define _PASS_NAME_NAME			_GCC_PLUGIN_STRINGIFY(PASS_NAME)
+
+#define __MAKE_PASS_NAME_PASS(n)	_GCC_PLUGIN_CONCAT3(make_, n, _pass)
+#define _MAKE_PASS_NAME_PASS		__MAKE_PASS_NAME_PASS(PASS_NAME)
+
+#ifdef NO_GATE
+#define _GATE NULL
+#define _HAS_GATE false
+#else
+#define __GATE(n)			_GCC_PLUGIN_CONCAT2(n, _gate)
+#define _GATE				__GATE(PASS_NAME)
+#define _HAS_GATE true
+#endif
+
+#ifdef NO_EXECUTE
+#define _EXECUTE NULL
+#define _HAS_EXECUTE false
+#else
+#define __EXECUTE(n)			_GCC_PLUGIN_CONCAT2(n, _execute)
+#define _EXECUTE			__EXECUTE(PASS_NAME)
+#define _HAS_EXECUTE true
+#endif
+
+#ifndef PROPERTIES_REQUIRED
+#define PROPERTIES_REQUIRED 0
+#endif
+
+#ifndef PROPERTIES_PROVIDED
+#define PROPERTIES_PROVIDED 0
+#endif
+
+#ifndef PROPERTIES_DESTROYED
+#define PROPERTIES_DESTROYED 0
+#endif
+
+#ifndef TODO_FLAGS_START
+#define TODO_FLAGS_START 0
+#endif
+
+#ifndef TODO_FLAGS_FINISH
+#define TODO_FLAGS_FINISH 0
+#endif
+
+namespace {
+static const pass_data _PASS_NAME_PASS_DATA = {
+		.type			= SIMPLE_IPA_PASS,
+		.name			= _PASS_NAME_NAME,
+		.optinfo_flags		= OPTGROUP_NONE,
+		.tv_id			= TV_NONE,
+		.properties_required	= PROPERTIES_REQUIRED,
+		.properties_provided	= PROPERTIES_PROVIDED,
+		.properties_destroyed	= PROPERTIES_DESTROYED,
+		.todo_flags_start	= TODO_FLAGS_START,
+		.todo_flags_finish	= TODO_FLAGS_FINISH,
+};
+
+class _PASS_NAME_PASS : public simple_ipa_opt_pass {
+public:
+	_PASS_NAME_PASS() : simple_ipa_opt_pass(_PASS_NAME_PASS_DATA, g) {}
+
+#ifndef NO_GATE
+	virtual bool gate(function *) { return _GATE(); }
+#endif
+
+	virtual opt_pass *clone() { return new _PASS_NAME_PASS(); }
+
+#ifndef NO_EXECUTE
+	virtual unsigned int execute(function *) { return _EXECUTE(); }
+#endif
+};
+}
+
+opt_pass *_MAKE_PASS_NAME_PASS(void)
+{
+	return new _PASS_NAME_PASS();
+}
+
+/* clean up user provided defines */
+#undef PASS_NAME
+#undef NO_GATE
+#undef NO_EXECUTE
+
+#undef PROPERTIES_DESTROYED
+#undef PROPERTIES_PROVIDED
+#undef PROPERTIES_REQUIRED
+#undef TODO_FLAGS_FINISH
+#undef TODO_FLAGS_START
+
+/* clean up generated defines */
+#undef _EXECUTE
+#undef __EXECUTE
+#undef _GATE
+#undef __GATE
+#undef _GCC_PLUGIN_CONCAT2
+#undef _GCC_PLUGIN_CONCAT3
+#undef _GCC_PLUGIN_STRINGIFY
+#undef __GCC_PLUGIN_STRINGIFY
+#undef _HAS_EXECUTE
+#undef _HAS_GATE
+#undef _MAKE_PASS_NAME_PASS
+#undef __MAKE_PASS_NAME_PASS
+#undef _PASS_NAME_NAME
+#undef _PASS_NAME_PASS
+#undef __PASS_NAME_PASS
+#undef _PASS_NAME_PASS_DATA
+#undef __PASS_NAME_PASS_DATA
+
+#endif /* PASS_NAME */