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, 174 insertions, 0 deletions

diff --git a/scripts/check-sysctl-docs b/scripts/check-sysctl-docs
new file mode 100755
index 000000000..910fd8a9a
--- /dev/null
+++ b/scripts/check-sysctl-docs
@@ -0,0 +1,174 @@
+#!/usr/bin/env -S gawk -f
+# SPDX-License-Identifier: GPL-2.0
+
+# Script to check sysctl documentation against source files
+#
+# Copyright (c) 2020 Stephen Kitt
+
+# Example invocation:
+#	scripts/check-sysctl-docs -vtable="kernel" \
+#		Documentation/admin-guide/sysctl/kernel.rst \
+#		$(git grep -l register_sysctl)
+#
+# Specify -vdebug=1 to see debugging information
+
+BEGIN {
+	if (!table) {
+	print "Please specify the table to look for using the table variable" > "/dev/stderr"
+	exit 1
+	}
+
+	# Documentation title skiplist
+	skiplist[0] = "^Documentation for"
+	skiplist[1] = "Network core options$"
+	skiplist[2] = "POSIX message queues filesystem$"
+	skiplist[3] = "Configuration options"
+	skiplist[4] = ". /proc/sys/fs"
+	skiplist[5] = "^Introduction$"
+	skiplist[6] = "^seccomp$"
+	skiplist[7] = "^pty$"
+	skiplist[8] = "^firmware_config$"
+	skiplist[9] = "^random$"
+}
+
+# The following globals are used:
+# documented: maps documented entries (each key is an entry)
+# entries: maps ctl_table names and procnames to counts (so
+#          enumerating the subkeys for a given ctl_table lists its
+#          procnames)
+# curtable: the name of the current ctl_table struct
+# curentry: the name of the current proc entry (procname when parsing
+#           a ctl_table, constructed path when parsing a ctl_path)
+
+
+# Remove punctuation from the given value
+function trimpunct(value) {
+	while (value ~ /^["&]/) {
+		value = substr(value, 2)
+	}
+	while (value ~ /[]["&,}]$/) {
+		value = substr(value, 1, length(value) - 1)
+	}
+	return value
+}
+
+# Print the information for the given entry
+function printentry(entry) {
+	seen[entry]++
+	printf "* %s from %s", entry, file[entry]
+	if (documented[entry]) {
+		printf " (documented)"
+	}
+	print ""
+}
+
+
+# Stage 1: build the list of documented entries
+FNR == NR && /^=+$/ {
+	for (i in skiplist) {
+		if (prevline ~ skiplist[i]) {
+			next
+		}
+	}
+
+	# The previous line is a section title, parse it
+	$0 = prevline
+	if (debug) print "Parsing " $0
+	inbrackets = 0
+	for (i = 1; i <= NF; i++) {
+		if (length($i) == 0) {
+			continue
+		}
+		if (!inbrackets && substr($i, 1, 1) == "(") {
+			inbrackets = 1
+		}
+		if (!inbrackets) {
+			token = trimpunct($i)
+			if (length(token) > 0 && token != "and") {
+				if (debug) print trimpunct($i)
+					documented[trimpunct($i)]++
+			}
+		}
+		if (inbrackets && substr($i, length($i), 1) == ")") {
+			inbrackets = 0
+		}
+	}
+}
+
+FNR == NR {
+	prevline = $0
+	next
+}
+
+
+# Stage 2: process each file and find all sysctl tables
+BEGINFILE {
+	delete entries
+	curtable = ""
+	curentry = ""
+	delete vars
+	if (debug) print "Processing file " FILENAME
+}
+
+/^static( const)? struct ctl_table/ {
+	match($0, /static( const)? struct ctl_table ([^][]+)/, tables)
+	curtable = tables[2]
+	if (debug) print "Processing table " curtable
+}
+
+/^};$/ {
+	curtable = ""
+	curentry = ""
+	delete vars
+}
+
+curtable && /\.procname[\t ]*=[\t ]*".+"/ {
+	match($0, /.procname[\t ]*=[\t ]*"([^"]+)"/, names)
+	curentry = names[1]
+	if (debug) print "Adding entry " curentry " to table " curtable
+	entries[curtable][curentry]++
+	file[curentry] = FILENAME
+}
+
+curtable && /UCOUNT_ENTRY.*/ {
+	match($0, /UCOUNT_ENTRY\("([^"]+)"\)/, names)
+	curentry = names[1]
+	if (debug) print "Adding entry " curentry " to table " curtable
+	entries[curtable][curentry]++
+	file[curentry] = FILENAME
+}
+
+/register_sysctl.*/ {
+	match($0, /register_sysctl(|_init|_sz)\("([^"]+)" *, *([^,)]+)/, tables)
+	if (debug) print "Registering table " tables[3] " at " tables[2]
+	if (tables[2] == table) {
+		for (entry in entries[tables[3]]) {
+			printentry(entry)
+		}
+	}
+}
+
+/kmemdup.*/ {
+	match($0, /([^ \t]+) *= *kmemdup\(([^,]+) *,/, names)
+	if (debug) print "Found variable " names[1] " for table " names[2]
+	if (names[2] in entries) {
+		vars[names[1]] = names[2]
+	}
+}
+
+/__register_sysctl_table.*/ {
+	match($0, /__register_sysctl_table\([^,]+, *"([^"]+)" *, *([^,]+)/, tables)
+	if (debug) print "Registering variable table " tables[2] " at " tables[1]
+	if (tables[1] == table && tables[2] in vars) {
+		for (entry in entries[vars[tables[2]]]) {
+			printentry(entry)
+		}
+	}
+}
+
+END {
+	for (entry in documented) {
+		if (!seen[entry])
+			print "No implementation for " entry
+	}
+}