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, 173 insertions, 0 deletions

diff --git a/Documentation/sphinx/kernel_abi.py b/Documentation/sphinx/kernel_abi.py
new file mode 100644
index 000000000..5667f207d
--- /dev/null
+++ b/Documentation/sphinx/kernel_abi.py
@@ -0,0 +1,173 @@
+# -*- coding: utf-8; mode: python -*-
+# coding=utf-8
+# SPDX-License-Identifier: GPL-2.0
+#
+"""
+    kernel-abi
+    ~~~~~~~~~~
+
+    Implementation of the ``kernel-abi`` reST-directive.
+
+    :copyright:  Copyright (C) 2016  Markus Heiser
+    :copyright:  Copyright (C) 2016-2020  Mauro Carvalho Chehab
+    :maintained-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
+    :license:    GPL Version 2, June 1991 see Linux/COPYING for details.
+
+    The ``kernel-abi`` (:py:class:`KernelCmd`) directive calls the
+    AbiParser class to parse the Kernel ABI files.
+
+    Overview of directive's argument and options.
+
+    .. code-block:: rst
+
+        .. kernel-abi:: <ABI directory location>
+            :debug:
+
+    The argument ``<ABI directory location>`` is required. It contains the
+    location of the ABI files to be parsed.
+
+    ``debug``
+      Inserts a code-block with the *raw* reST. Sometimes it is helpful to see
+      what reST is generated.
+
+"""
+
+import os
+import re
+import sys
+
+from docutils import nodes, statemachine
+from docutils.statemachine import ViewList
+from docutils.parsers.rst import directives, Directive
+from sphinx.util.docutils import switch_source_input
+from sphinx.util import logging
+
+srctree = os.path.abspath(os.environ["srctree"])
+sys.path.insert(0, os.path.join(srctree, "tools/lib/python"))
+
+from abi.abi_parser import AbiParser
+
+__version__ = "1.0"
+
+logger = logging.getLogger('kernel_abi')
+path = os.path.join(srctree, "Documentation/ABI")
+
+_kernel_abi = None
+
+def get_kernel_abi():
+    """
+    Initialize kernel_abi global var, if not initialized yet.
+
+    This is needed to avoid warnings during Sphinx module initialization.
+    """
+    global _kernel_abi
+
+    if not _kernel_abi:
+        # Parse ABI symbols only once
+        _kernel_abi = AbiParser(path, logger=logger)
+        _kernel_abi.parse_abi()
+        _kernel_abi.check_issues()
+
+    return _kernel_abi
+
+def setup(app):
+
+    app.add_directive("kernel-abi", KernelCmd)
+    return {
+        "version": __version__,
+        "parallel_read_safe": True,
+        "parallel_write_safe": True
+    }
+
+
+class KernelCmd(Directive):
+    """KernelABI (``kernel-abi``) directive"""
+
+    required_arguments = 1
+    optional_arguments = 3
+    has_content = False
+    final_argument_whitespace = True
+    parser = None
+
+    option_spec = {
+        "debug": directives.flag,
+        "no-symbols": directives.flag,
+        "no-files":  directives.flag,
+    }
+
+    def run(self):
+        kernel_abi = get_kernel_abi()
+
+        doc = self.state.document
+        if not doc.settings.file_insertion_enabled:
+            raise self.warning("docutils: file insertion disabled")
+
+        env = self.state.document.settings.env
+        content = ViewList()
+        node = nodes.section()
+
+        abi_type = self.arguments[0]
+
+        if "no-symbols" in self.options:
+            show_symbols = False
+        else:
+            show_symbols = True
+
+        if "no-files" in self.options:
+            show_file = False
+        else:
+            show_file = True
+
+        tab_width = self.options.get('tab-width',
+                                     self.state.document.settings.tab_width)
+
+        old_f = None
+        n = 0
+        n_sym = 0
+        for msg, f, ln in kernel_abi.doc(show_file=show_file,
+                                            show_symbols=show_symbols,
+                                            filter_path=abi_type):
+            n_sym += 1
+            msg_list = statemachine.string2lines(msg, tab_width,
+                                                 convert_whitespace=True)
+            if "debug" in self.options:
+                lines = [
+                    "", "",  ".. code-block:: rst",
+                    "    :linenos:", ""
+                ]
+                for m in msg_list:
+                    lines.append("    " + m)
+            else:
+                lines = msg_list
+
+            for line in lines:
+                # sphinx counts lines from 0
+                content.append(line, f, ln - 1)
+                n += 1
+
+            if f != old_f:
+                # Add the file to Sphinx build dependencies if the file exists
+                fname = os.path.join(srctree, f)
+                if os.path.isfile(fname):
+                    env.note_dependency(fname)
+
+                old_f = f
+
+            # Sphinx doesn't like to parse big messages. So, let's
+            # add content symbol by symbol
+            if content:
+                self.do_parse(content, node)
+                content = ViewList()
+
+        if show_symbols and not show_file:
+            logger.verbose("%s ABI: %i symbols (%i ReST lines)" % (abi_type, n_sym, n))
+        elif not show_symbols and show_file:
+            logger.verbose("%s ABI: %i files (%i ReST lines)" % (abi_type, n_sym, n))
+        else:
+            logger.verbose("%s ABI: %i data (%i ReST lines)" % (abi_type, n_sym, n))
+
+        return node.children
+
+    def do_parse(self, content, node):
+        with switch_source_input(self.state, content):
+            self.state.nested_parse(content, 0, node, match_titles=1)