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, 92 insertions, 0 deletions

diff --git a/arch/arm/boot/Makefile b/arch/arm/boot/Makefile
new file mode 100644
index 000000000..ba9b9a802
--- /dev/null
+++ b/arch/arm/boot/Makefile
@@ -0,0 +1,92 @@
+#
+# arch/arm/boot/Makefile
+#
+# This file is included by the global makefile so that you can add your own
+# architecture-specific flags and dependencies.
+#
+# 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) 1995-2002 Russell King
+#
+OBJCOPYFLAGS	:=-O binary -R .comment -S
+
+#   ZRELADDR == virt_to_phys(PAGE_OFFSET + TEXT_OFFSET)
+ifdef CONFIG_PHYS_OFFSET
+add_hex = $(shell printf 0x%x $$(( $(1) + $(2) )) )
+ZRELADDR    := $(call add_hex, $(CONFIG_PHYS_OFFSET), $(TEXT_OFFSET))
+endif
+
+PHYS_OFFSET := $(CONFIG_PHYS_OFFSET)
+export ZRELADDR PARAMS_PHYS PHYS_OFFSET
+
+targets := Image zImage xipImage bootpImage uImage
+
+ifeq ($(CONFIG_XIP_KERNEL),y)
+
+cmd_deflate_xip_data = $(CONFIG_SHELL) -c '$(src)/deflate_xip_data.sh $< $@'
+
+ifeq ($(CONFIG_XIP_DEFLATED_DATA),y)
+quiet_cmd_mkxip = XIPZ    $@
+cmd_mkxip = $(cmd_objcopy) && $(cmd_deflate_xip_data)
+else
+quiet_cmd_mkxip = $(quiet_cmd_objcopy)
+cmd_mkxip = $(cmd_objcopy)
+endif
+
+$(obj)/xipImage: vmlinux FORCE
+	$(call if_changed,mkxip)
+	@$(kecho) '  Physical Address of xipImage: $(CONFIG_XIP_PHYS_ADDR)'
+
+$(obj)/Image $(obj)/zImage: FORCE
+	@echo 'Kernel configured for XIP (CONFIG_XIP_KERNEL=y)'
+	@echo 'Only the xipImage target is available in this case'
+	@false
+
+else
+
+$(obj)/xipImage: FORCE
+	@echo 'Kernel not configured for XIP (CONFIG_XIP_KERNEL!=y)'
+	@false
+
+$(obj)/Image: vmlinux FORCE
+	$(call if_changed,objcopy)
+
+$(obj)/compressed/vmlinux: $(obj)/Image FORCE
+	$(Q)$(MAKE) $(build)=$(obj)/compressed $@
+
+$(obj)/zImage:	$(obj)/compressed/vmlinux FORCE
+	$(call if_changed,objcopy)
+
+endif
+
+ifneq ($(LOADADDR),)
+  UIMAGE_LOADADDR=$(LOADADDR)
+else
+  ifeq ($(CONFIG_ZBOOT_ROM),y)
+    UIMAGE_LOADADDR=$(CONFIG_ZBOOT_ROM_TEXT)
+  else
+    UIMAGE_LOADADDR=$(ZRELADDR)
+  endif
+endif
+
+check_for_multiple_loadaddr = \
+if [ $(words $(UIMAGE_LOADADDR)) -ne 1 ]; then \
+	echo 'multiple (or no) load addresses: $(UIMAGE_LOADADDR)'; \
+	echo 'This is incompatible with uImages'; \
+	echo 'Specify LOADADDR on the commandline to build an uImage'; \
+	false; \
+fi
+
+$(obj)/uImage:	$(obj)/zImage FORCE
+	@$(check_for_multiple_loadaddr)
+	$(call if_changed,uimage)
+
+$(obj)/bootp/bootp: $(obj)/zImage FORCE
+	$(Q)$(MAKE) $(build)=$(obj)/bootp $@
+
+$(obj)/bootpImage: $(obj)/bootp/bootp FORCE
+	$(call if_changed,objcopy)
+
+subdir-	    := bootp compressed dts