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, 165 insertions, 0 deletions

diff --git a/scripts/Makefile.host b/scripts/Makefile.host
new file mode 100644
index 000000000..c1dedf646
--- /dev/null
+++ b/scripts/Makefile.host
@@ -0,0 +1,165 @@
+# SPDX-License-Identifier: GPL-2.0
+
+# LEX
+# ---------------------------------------------------------------------------
+quiet_cmd_flex = LEX     $@
+      cmd_flex = $(LEX) -o$@ -L $<
+
+$(obj)/%.lex.c: $(src)/%.l FORCE
+	$(call if_changed,flex)
+
+# YACC
+# ---------------------------------------------------------------------------
+quiet_cmd_bison = YACC    $(basename $@).[ch]
+      cmd_bison = $(YACC) -o $(basename $@).c --defines=$(basename $@).h -t -l $<
+
+$(obj)/%.tab.c $(obj)/%.tab.h: $(src)/%.y FORCE
+	$(call if_changed,bison)
+
+# ==========================================================================
+# Building binaries on the host system
+# Binaries are used during the compilation of the kernel, for example
+# to preprocess a data file.
+#
+# Both C and C++ are supported, but preferred language is C for such utilities.
+# Rust is also supported, but it may only be used in scenarios where a Rust
+# toolchain is required to be available (e.g. when  `CONFIG_RUST` is enabled).
+#
+# Sample syntax (see Documentation/kbuild/makefiles.rst for reference)
+# hostprogs := bin2hex
+# Will compile bin2hex.c and create an executable named bin2hex
+#
+# hostprogs     := lxdialog
+# lxdialog-objs := checklist.o lxdialog.o
+# Will compile lxdialog.c and checklist.c, and then link the executable
+# lxdialog, based on checklist.o and lxdialog.o
+#
+# hostprogs       := qconf
+# qconf-cxxobjs   := qconf.o
+# qconf-objs      := menu.o
+# Will compile qconf as a C++ program, and menu as a C program.
+# They are linked as C++ code to the executable qconf
+#
+# hostprogs   := target
+# target-rust := y
+# Will compile `target` as a Rust program, using `target.rs` as the crate root.
+# The crate may consist of several source files.
+
+# C code
+# Executables compiled from a single .c file
+host-csingle	:= $(foreach m,$(hostprogs), \
+			$(if $($(m)-objs)$($(m)-cxxobjs)$($(m)-rust),,$(m)))
+
+# C executables linked based on several .o files
+host-cmulti	:= $(foreach m,$(hostprogs),\
+		   $(if $($(m)-cxxobjs)$($(m)-rust),,$(if $($(m)-objs),$(m))))
+
+# Object (.o) files compiled from .c files
+host-cobjs	:= $(sort $(foreach m,$(hostprogs),$($(m)-objs)))
+
+# C++ code
+# C++ executables compiled from at least one .cc file
+# and zero or more .c files
+host-cxxmulti	:= $(foreach m,$(hostprogs),$(if $($(m)-cxxobjs),$(m)))
+
+# C++ Object (.o) files compiled from .cc files
+host-cxxobjs	:= $(sort $(foreach m,$(host-cxxmulti),$($(m)-cxxobjs)))
+
+# Rust code
+# Executables compiled from a single Rust crate (which may consist of
+# one or more .rs files)
+host-rust	:= $(foreach m,$(hostprogs),$(if $($(m)-rust),$(m)))
+
+host-csingle	:= $(addprefix $(obj)/,$(host-csingle))
+host-cmulti	:= $(addprefix $(obj)/,$(host-cmulti))
+host-cobjs	:= $(addprefix $(obj)/,$(host-cobjs))
+host-cxxmulti	:= $(addprefix $(obj)/,$(host-cxxmulti))
+host-cxxobjs	:= $(addprefix $(obj)/,$(host-cxxobjs))
+host-rust	:= $(addprefix $(obj)/,$(host-rust))
+
+#####
+# Handle options to gcc. Support building with separate output directory
+
+hostc_flags    = -Wp,-MMD,$(depfile) \
+                 $(KBUILD_HOSTCFLAGS) $(HOST_EXTRACFLAGS) \
+                 $(HOSTCFLAGS_$(target-stem).o)
+hostcxx_flags  = -Wp,-MMD,$(depfile) \
+                 $(KBUILD_HOSTCXXFLAGS) $(HOST_EXTRACXXFLAGS) \
+                 $(HOSTCXXFLAGS_$(target-stem).o)
+
+# `--out-dir` is required to avoid temporaries being created by `rustc` in the
+# current working directory, which may be not accessible in the out-of-tree
+# modules case.
+hostrust_flags = --out-dir $(dir $@) --emit=dep-info=$(depfile) \
+		 -Clinker-flavor=gcc -Clinker=$(HOSTCC) \
+		 -Clink-args='$(call escsq,$(KBUILD_HOSTLDFLAGS))' \
+                 $(KBUILD_HOSTRUSTFLAGS) $(HOST_EXTRARUSTFLAGS) \
+                 $(HOSTRUSTFLAGS_$(target-stem))
+
+# $(obj) for including generated headers from checkin source files
+ifdef building_out_of_srctree
+hostc_flags   += -I $(obj)
+hostcxx_flags += -I $(obj)
+endif
+
+#####
+# Compile programs on the host
+
+# Create executable from a single .c file
+# host-csingle -> Executable
+quiet_cmd_host-csingle 	= HOSTCC  $@
+      cmd_host-csingle	= $(HOSTCC) $(hostc_flags) $(KBUILD_HOSTLDFLAGS) -o $@ $< \
+		$(KBUILD_HOSTLDLIBS) $(HOSTLDLIBS_$(target-stem))
+$(host-csingle): $(obj)/%: $(obj)/%.c FORCE
+	$(call if_changed_dep,host-csingle)
+
+# Link an executable based on list of .o files, all plain c
+# host-cmulti -> executable
+quiet_cmd_host-cmulti	= HOSTLD  $@
+      cmd_host-cmulti	= $(HOSTCC) $(KBUILD_HOSTLDFLAGS) -o $@ \
+			  $(addprefix $(obj)/, $($(target-stem)-objs)) \
+			  $(KBUILD_HOSTLDLIBS) $(HOSTLDLIBS_$(target-stem))
+$(host-cmulti): FORCE
+	$(call if_changed,host-cmulti)
+$(call multi_depend, $(host-cmulti), , -objs)
+
+# Create .o file from a single .c file
+# host-cobjs -> .o
+quiet_cmd_host-cobjs	= HOSTCC  $@
+      cmd_host-cobjs	= $(HOSTCC) $(hostc_flags) -c -o $@ $<
+$(host-cobjs): $(obj)/%.o: $(obj)/%.c FORCE
+	$(call if_changed_dep,host-cobjs)
+
+# Link an executable based on list of .o files, a mixture of .c and .cc
+# host-cxxmulti -> executable
+quiet_cmd_host-cxxmulti	= HOSTLD  $@
+      cmd_host-cxxmulti	= $(HOSTCXX) $(KBUILD_HOSTLDFLAGS) -o $@ \
+			  $(foreach o,objs cxxobjs,\
+			  $(addprefix $(obj)/, $($(target-stem)-$(o)))) \
+			  $(KBUILD_HOSTLDLIBS) $(HOSTLDLIBS_$(target-stem))
+$(host-cxxmulti): FORCE
+	$(call if_changed,host-cxxmulti)
+$(call multi_depend, $(host-cxxmulti), , -objs -cxxobjs)
+
+# Create .o file from a single .cc (C++) file
+quiet_cmd_host-cxxobjs	= HOSTCXX $@
+      cmd_host-cxxobjs	= $(HOSTCXX) $(hostcxx_flags) -c -o $@ $<
+$(host-cxxobjs): $(obj)/%.o: $(obj)/%.cc FORCE
+	$(call if_changed_dep,host-cxxobjs)
+
+# Create executable from a single Rust crate (which may consist of
+# one or more `.rs` files)
+# host-rust -> Executable
+quiet_cmd_host-rust	= HOSTRUSTC $@
+      cmd_host-rust	= \
+	$(HOSTRUSTC) $(hostrust_flags) --emit=link=$@ $<
+$(host-rust): $(obj)/%: $(src)/%.rs FORCE
+	+$(call if_changed_dep,host-rust)
+
+targets += $(host-csingle) $(host-cmulti) $(host-cobjs) \
+	   $(host-cxxmulti) $(host-cxxobjs) $(host-rust)
+
+# %.lex.o <- %.lex.c <- %.l
+# %.tab.o <- %.tab.[ch] <- %.y
+targets += $(call intermediate_targets, .lex.o, .lex.c) \
+           $(call intermediate_targets, .tab.o, .tab.c .tab.h)