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>

4 files changed, 151 insertions, 0 deletions

diff --git a/arch/xtensa/boot/boot-elf/.gitignore b/arch/xtensa/boot/boot-elf/.gitignore
new file mode 100644
index 000000000..747340450
--- /dev/null
+++ b/arch/xtensa/boot/boot-elf/.gitignore
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+boot.lds
diff --git a/arch/xtensa/boot/boot-elf/Makefile b/arch/xtensa/boot/boot-elf/Makefile
new file mode 100644
index 000000000..faec20029
--- /dev/null
+++ b/arch/xtensa/boot/boot-elf/Makefile
@@ -0,0 +1,30 @@
+#
+# This file is subject to the terms and conditions of the GNU General Public
+# License.  See the file "COPYING" in the main directory of this archive
+# for more details.
+#
+
+OBJCOPY_ARGS := -O $(if $(CONFIG_CPU_BIG_ENDIAN),elf32-xtensa-be,elf32-xtensa-le)
+
+CPPFLAGS_boot.lds += -P -C
+KBUILD_AFLAGS += -mtext-section-literals
+
+boot-y		:= bootstrap.o
+targets		+= $(boot-y) boot.lds
+
+OBJS		:= $(addprefix $(obj)/,$(boot-y))
+
+$(obj)/Image.o: $(obj)/../vmlinux.bin $(OBJS)
+	$(Q)$(OBJCOPY) $(OBJCOPY_ARGS) -R .comment \
+		--add-section image=$< \
+		--set-section-flags image=contents,alloc,load,load,data \
+		$(OBJS) $@
+
+$(obj)/../Image.elf: $(obj)/Image.o $(obj)/boot.lds
+	$(Q)$(LD) $(KBUILD_LDFLAGS) \
+		-T $(obj)/boot.lds \
+		--build-id=none \
+		-o $@ $(obj)/Image.o
+	$(Q)$(kecho) '  Kernel: $@ is ready'
+
+all Image: $(obj)/../Image.elf
diff --git a/arch/xtensa/boot/boot-elf/boot.lds.S b/arch/xtensa/boot/boot-elf/boot.lds.S
new file mode 100644
index 000000000..32a3b7c5b
--- /dev/null
+++ b/arch/xtensa/boot/boot-elf/boot.lds.S
@@ -0,0 +1,40 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ *  linux/arch/xtensa/boot/boot-elf/boot.lds.S
+ *
+ *  Copyright (C) 2008 - 2013 by Tensilica Inc.
+ *
+ *  Chris Zankel <chris@zankel.net>
+ *  Marc Gauthier <marc@tensilica.com
+ *  Pete Delaney <piet@tensilica.com>
+ */
+
+#include <asm/vectors.h>
+OUTPUT_ARCH(xtensa)
+ENTRY(_ResetVector)
+
+SECTIONS
+{
+	.ResetVector.text XCHAL_RESET_VECTOR_VADDR :
+	{
+		*(.ResetVector.text)
+	}
+
+	.image KERNELOFFSET: AT (CONFIG_KERNEL_LOAD_ADDRESS)
+	{
+		_image_start = .;
+		*(image)
+		. = (. + 3) & ~ 3;
+		_image_end = .	;
+	}
+
+	.bss ((LOADADDR(.image) + SIZEOF(.image) + 3) & ~ 3):
+	{
+		__bss_start = .;
+		*(.sbss)
+		*(.scommon)
+		*(.dynbss)
+		*(.bss)
+		__bss_end = .;
+	}
+}
diff --git a/arch/xtensa/boot/boot-elf/bootstrap.S b/arch/xtensa/boot/boot-elf/bootstrap.S
new file mode 100644
index 000000000..2dd28931d
--- /dev/null
+++ b/arch/xtensa/boot/boot-elf/bootstrap.S
@@ -0,0 +1,79 @@
+/*
+ * arch/xtensa/boot/boot-elf/bootstrap.S
+ *
+ * Low-level exception handling
+ *
+ * This file is subject to the terms and conditions of the GNU General Public
+ * License.  See the file "COPYING" in the main directory of this archive
+ * for more details.
+ *
+ * Copyright (C) 2004 - 2013 by Tensilica Inc.
+ *
+ * Chris Zankel <chris@zankel.net>
+ * Marc Gauthier <marc@tensilica.com>
+ * Piet Delaney <piet@tensilica.com>
+ */
+
+#include <asm/bootparam.h>
+#include <asm/initialize_mmu.h>
+#include <asm/vectors.h>
+#include <linux/linkage.h>
+
+	.section	.ResetVector.text, "ax"
+	.global         _ResetVector
+	.global         reset
+
+_ResetVector:
+	_j _SetupMMU
+
+	.begin  no-absolute-literals
+	.literal_position
+
+#ifdef CONFIG_PARSE_BOOTPARAM
+	.align 4
+_bootparam:
+	.short	BP_TAG_FIRST
+	.short	4
+	.long	BP_VERSION
+	.short	BP_TAG_LAST
+	.short	0
+	.long	0
+#endif
+
+	.align  4
+_SetupMMU:
+#if XCHAL_HAVE_WINDOWED
+	movi	a0, 0
+	wsr	a0, windowbase
+	rsync
+	movi	a0, 1
+	wsr	a0, windowstart
+	rsync
+#endif
+	movi	a0, 0x1F
+	wsr	a0, ps
+	rsync
+
+#ifndef CONFIG_INITIALIZE_XTENSA_MMU_INSIDE_VMLINUX
+	initialize_mmu
+#endif
+
+	rsil    a0, XCHAL_DEBUGLEVEL-1
+	rsync
+reset:
+#if defined(CONFIG_INITIALIZE_XTENSA_MMU_INSIDE_VMLINUX) && \
+	XCHAL_HAVE_PTP_MMU && XCHAL_HAVE_SPANNING_WAY
+	movi	a0, CONFIG_KERNEL_LOAD_ADDRESS
+#else
+	movi	a0, KERNELOFFSET
+#endif
+#ifdef CONFIG_PARSE_BOOTPARAM
+	movi	a2, _bootparam
+#else
+	movi	a2, 0
+#endif
+	movi	a3, 0
+	movi	a4, 0
+	jx      a0
+
+	.end    no-absolute-literals