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, 107 insertions, 0 deletions

diff --git a/tools/perf/util/setup.py b/tools/perf/util/setup.py
new file mode 100644
index 000000000..a0ce76624
--- /dev/null
+++ b/tools/perf/util/setup.py
@@ -0,0 +1,107 @@
+from os import getenv, path
+from subprocess import Popen, PIPE
+from re import sub
+import shlex
+
+cc = getenv("CC")
+assert cc, "Environment variable CC not set"
+
+# Check if CC has options, as is the case in yocto, where it uses CC="cc --sysroot..."
+cc_tokens = cc.split()
+if len(cc_tokens) > 1:
+    cc = cc_tokens[0]
+    cc_options = " ".join([str(e) for e in cc_tokens[1:]]) + " "
+else:
+    cc_options = ""
+
+# ignore optional stderr could be None as it is set to PIPE to avoid that.
+# mypy: disable-error-code="union-attr"
+cc_is_clang = b"clang version" in Popen([cc, "-v"], stderr=PIPE).stderr.readline()
+
+srctree = getenv('srctree')
+assert srctree, "Environment variable srctree, for the Linux sources, not set"
+src_feature_tests  = f'{srctree}/tools/build/feature'
+
+def clang_has_option(option):
+    error_substrings = (
+        b"unknown argument",
+        b"is not supported",
+        b"unknown warning option"
+    )
+    cmd = shlex.split(f"{cc} {cc_options} {option}") + [
+        "-o", "/dev/null",
+        path.join(src_feature_tests, "test-hello.c")
+    ]
+    cc_output = Popen(cmd, stderr=PIPE).stderr.readlines()
+    return not any(any(error in line for error in error_substrings) for line in cc_output)
+
+if cc_is_clang:
+    from sysconfig import get_config_vars
+    vars = get_config_vars()
+    for var in ('CFLAGS', 'OPT'):
+        vars[var] = sub("-specs=[^ ]+", "", vars[var])
+        if not clang_has_option("-mcet"):
+            vars[var] = sub("-mcet", "", vars[var])
+        if not clang_has_option("-fcf-protection"):
+            vars[var] = sub("-fcf-protection", "", vars[var])
+        if not clang_has_option("-fstack-clash-protection"):
+            vars[var] = sub("-fstack-clash-protection", "", vars[var])
+        if not clang_has_option("-fstack-protector-strong"):
+            vars[var] = sub("-fstack-protector-strong", "", vars[var])
+        if not clang_has_option("-fno-semantic-interposition"):
+            vars[var] = sub("-fno-semantic-interposition", "", vars[var])
+        if not clang_has_option("-ffat-lto-objects"):
+            vars[var] = sub("-ffat-lto-objects", "", vars[var])
+        if not clang_has_option("-ftree-loop-distribute-patterns"):
+            vars[var] = sub("-ftree-loop-distribute-patterns", "", vars[var])
+        if not clang_has_option("-gno-variable-location-views"):
+            vars[var] = sub("-gno-variable-location-views", "", vars[var])
+
+from setuptools import setup, Extension
+
+from setuptools.command.build_ext   import build_ext   as _build_ext
+from setuptools.command.install_lib import install_lib as _install_lib
+
+class build_ext(_build_ext):
+    def finalize_options(self):
+        _build_ext.finalize_options(self)
+        self.build_lib  = build_lib
+        self.build_temp = build_tmp
+
+class install_lib(_install_lib):
+    def finalize_options(self):
+        _install_lib.finalize_options(self)
+        self.build_dir = build_lib
+
+
+# switch off several checks (need to be at the end of cflags list)
+extra_cflags = ['-fno-strict-aliasing', '-Wno-write-strings', '-Wno-unused-parameter', '-Wno-redundant-decls' ]
+if cc_is_clang:
+    extra_cflags += ["-Wno-unused-command-line-argument" ]
+    if clang_has_option("-Wno-cast-function-type-mismatch"):
+        extra_cflags += ["-Wno-cast-function-type-mismatch" ]
+else:
+    extra_cflags += ['-Wno-cast-function-type' ]
+
+# The python headers have mixed code with declarations (decls after asserts, for instance)
+extra_cflags += [ "-Wno-declaration-after-statement" ]
+
+src_perf  = f'{srctree}/tools/perf'
+build_lib = getenv('PYTHON_EXTBUILD_LIB')
+build_tmp = getenv('PYTHON_EXTBUILD_TMP')
+
+perf = Extension('perf',
+                 sources = [ src_perf + '/util/python.c' ],
+		         include_dirs = ['util/include'],
+		         extra_compile_args = extra_cflags,
+                 )
+
+setup(name='perf',
+      version='0.1',
+      description='Interface with the Linux profiling infrastructure',
+      author='Arnaldo Carvalho de Melo',
+      author_email='acme@redhat.com',
+      license='GPLv2',
+      url='http://perf.wiki.kernel.org',
+      ext_modules=[perf],
+      cmdclass={'build_ext': build_ext, 'install_lib': install_lib})