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, 167 insertions, 0 deletions

diff --git a/scripts/Makefile.modinst b/scripts/Makefile.modinst
new file mode 100644
index 000000000..9ba45e5b3
--- /dev/null
+++ b/scripts/Makefile.modinst
@@ -0,0 +1,167 @@
+# SPDX-License-Identifier: GPL-2.0
+# ==========================================================================
+# Installing modules
+# ==========================================================================
+
+PHONY := __modinst
+__modinst:
+
+include $(objtree)/include/config/auto.conf
+include $(srctree)/scripts/Kbuild.include
+
+install-y :=
+
+ifeq ($(KBUILD_EXTMOD)$(sign-only),)
+
+# remove the old directory and symlink
+$(shell rm -fr $(MODLIB)/kernel $(MODLIB)/build)
+
+install-$(CONFIG_MODULES) += $(addprefix $(MODLIB)/, build modules.order)
+
+$(MODLIB)/build: FORCE
+	$(call cmd,symlink)
+
+quiet_cmd_symlink = SYMLINK $@
+      cmd_symlink = ln -s $(CURDIR) $@
+
+$(MODLIB)/modules.order: modules.order FORCE
+	$(call cmd,install_modorder)
+
+quiet_cmd_install_modorder = INSTALL $@
+      cmd_install_modorder = sed 's:^\(.*\)\.o$$:kernel/\1.ko:' $< > $@
+
+# Install modules.builtin(.modinfo,.ranges) even when CONFIG_MODULES is disabled.
+install-y += $(addprefix $(MODLIB)/, modules.builtin modules.builtin.modinfo)
+
+install-$(CONFIG_BUILTIN_MODULE_RANGES) += $(MODLIB)/modules.builtin.ranges
+
+$(addprefix $(MODLIB)/, modules.builtin modules.builtin.modinfo modules.builtin.ranges): $(MODLIB)/%: % FORCE
+	$(call cmd,install)
+
+endif
+
+modules := $(call read-file, modules.order)
+
+ifeq ($(KBUILD_EXTMOD),)
+dst := $(MODLIB)/kernel
+else
+INSTALL_MOD_DIR ?= updates
+dst := $(MODLIB)/$(INSTALL_MOD_DIR)
+endif
+
+$(foreach x, % :, $(if $(findstring $x, $(dst)), \
+	$(error module installation path cannot contain '$x')))
+
+suffix-y				:=
+ifdef CONFIG_MODULE_COMPRESS_ALL
+suffix-$(CONFIG_MODULE_COMPRESS_GZIP)	:= .gz
+suffix-$(CONFIG_MODULE_COMPRESS_XZ)	:= .xz
+suffix-$(CONFIG_MODULE_COMPRESS_ZSTD)	:= .zst
+endif
+
+modules := $(patsubst %.o, $(dst)/%.ko$(suffix-y), $(modules))
+install-$(CONFIG_MODULES) += $(modules)
+
+__modinst: $(install-y)
+	@:
+
+#
+# Installation
+#
+quiet_cmd_install = INSTALL $@
+      cmd_install = cp $< $@
+
+# Strip
+#
+# INSTALL_MOD_STRIP, if defined, will cause modules to be stripped after they
+# are installed. If INSTALL_MOD_STRIP is '1', then the default option
+# --strip-debug will be used. Otherwise, INSTALL_MOD_STRIP value will be used
+# as the options to the strip command.
+ifdef INSTALL_MOD_STRIP
+
+ifeq ($(INSTALL_MOD_STRIP),1)
+strip-option := --strip-debug
+else
+strip-option := $(INSTALL_MOD_STRIP)
+endif
+
+quiet_cmd_strip = STRIP   $@
+      cmd_strip = $(STRIP) $(strip-option) $@
+
+else
+
+quiet_cmd_strip =
+      cmd_strip = :
+
+endif
+
+#
+# Signing
+# Don't stop modules_install even if we can't sign external modules.
+#
+ifeq ($(filter pkcs11:%, $(CONFIG_MODULE_SIG_KEY)),)
+sig-key := $(if $(wildcard $(CONFIG_MODULE_SIG_KEY)),,$(objtree)/)$(CONFIG_MODULE_SIG_KEY)
+else
+sig-key := $(CONFIG_MODULE_SIG_KEY)
+endif
+quiet_cmd_sign = SIGN    $@
+      cmd_sign = $(objtree)/scripts/sign-file $(CONFIG_MODULE_SIG_HASH) "$(sig-key)" $(objtree)/certs/signing_key.x509 $@ \
+                 $(if $(KBUILD_EXTMOD),|| true)
+
+ifeq ($(sign-only),)
+
+# During modules_install, modules are signed only when CONFIG_MODULE_SIG_ALL=y.
+ifndef CONFIG_MODULE_SIG_ALL
+quiet_cmd_sign :=
+      cmd_sign := :
+endif
+
+# Create necessary directories
+$(foreach dir, $(sort $(dir $(install-y))), $(shell mkdir -p $(dir)))
+
+$(dst)/%.ko: %.ko FORCE
+	$(call cmd,install)
+	$(call cmd,strip)
+	$(call cmd,sign)
+
+ifdef CONFIG_MODULES
+__modinst: depmod
+
+PHONY += depmod
+depmod: $(install-y)
+	$(call cmd,depmod)
+
+quiet_cmd_depmod = DEPMOD  $(MODLIB)
+      cmd_depmod = $(srctree)/scripts/depmod.sh $(KERNELRELEASE)
+endif
+
+else
+
+$(dst)/%.ko: FORCE
+	$(call cmd,sign)
+
+endif
+
+#
+# Compression
+#
+quiet_cmd_gzip = GZIP    $@
+      cmd_gzip = $(KGZIP) -n -f $<
+quiet_cmd_xz = XZ      $@
+      cmd_xz = $(XZ) --check=crc32 --lzma2=dict=1MiB -f $<
+quiet_cmd_zstd = ZSTD    $@
+      cmd_zstd = $(ZSTD) --rm -f -q $<
+
+$(dst)/%.ko.gz: $(dst)/%.ko FORCE
+	$(call cmd,gzip)
+
+$(dst)/%.ko.xz: $(dst)/%.ko FORCE
+	$(call cmd,xz)
+
+$(dst)/%.ko.zst: $(dst)/%.ko FORCE
+	$(call cmd,zstd)
+
+PHONY += FORCE
+FORCE:
+
+.PHONY: $(PHONY)