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, 193 insertions, 0 deletions

diff --git a/scripts/headerdep.pl b/scripts/headerdep.pl
new file mode 100755
index 000000000..ebfcbef43
--- /dev/null
+++ b/scripts/headerdep.pl
@@ -0,0 +1,193 @@
+#! /usr/bin/env perl
+# SPDX-License-Identifier: GPL-2.0
+#
+# Detect cycles in the header file dependency graph
+# Vegard Nossum <vegardno@ifi.uio.no>
+#
+
+use strict;
+use warnings;
+
+use Getopt::Long;
+
+my $opt_all;
+my @opt_include;
+my $opt_graph;
+
+&Getopt::Long::Configure(qw(bundling pass_through));
+&GetOptions(
+	help	=> \&help,
+	version	=> \&version,
+
+	all	=> \$opt_all,
+	"I=s"	=> \@opt_include,
+	graph	=> \$opt_graph,
+);
+
+push @opt_include, 'include';
+my %deps = ();
+my %linenos = ();
+
+my @headers = grep { strip($_) } @ARGV;
+
+parse_all(@headers);
+
+if($opt_graph) {
+	graph();
+} else {
+	detect_cycles(@headers);
+}
+
+
+sub help {
+	print "Usage: $0 [options] file...\n";
+	print "\n";
+	print "Options:\n";
+	print "  --all\n";
+	print "  --graph\n";
+	print "\n";
+	print "  -I includedir\n";
+	print "\n";
+	print "To make nice graphs, try:\n";
+	print "  $0 --graph include/linux/kernel.h | dot -Tpng -o graph.png\n";
+	exit;
+}
+
+sub version {
+	print "headerdep version 2\n";
+	exit;
+}
+
+# Get a file name that is relative to our include paths
+sub strip {
+	my $filename = shift;
+
+	for my $i (@opt_include) {
+		my $stripped = $filename;
+		$stripped =~ s/^$i\///;
+
+		return $stripped if $stripped ne $filename;
+	}
+
+	return $filename;
+}
+
+# Search for the file name in the list of include paths
+sub search {
+	my $filename = shift;
+	return $filename if -f $filename;
+
+	for my $i (@opt_include) {
+		my $path = "$i/$filename";
+		return $path if -f $path;
+	}
+	return;
+}
+
+sub parse_all {
+	# Parse all the headers.
+	my @queue = @_;
+	while(@queue) {
+		my $header = pop @queue;
+		next if exists $deps{$header};
+
+		$deps{$header} = [] unless exists $deps{$header};
+
+		my $path = search($header);
+		next unless $path;
+
+		open(my $file, '<', $path) or die($!);
+		chomp(my @lines = <$file>);
+		close($file);
+
+		for my $i (0 .. $#lines) {
+			my $line = $lines[$i];
+			if(my($dep) = ($line =~ m/^#\s*include\s*<(.*?)>/)) {
+				push @queue, $dep;
+				push @{$deps{$header}}, [$i + 1, $dep];
+			}
+		}
+	}
+}
+
+sub print_cycle {
+	# $cycle[n] includes $cycle[n + 1];
+	# $cycle[-1] will be the culprit
+	my $cycle = shift;
+
+	# Adjust the line numbers
+	for my $i (0 .. $#$cycle - 1) {
+		$cycle->[$i]->[0] = $cycle->[$i + 1]->[0];
+	}
+	$cycle->[-1]->[0] = 0;
+
+	my $first = shift @$cycle;
+	my $last = pop @$cycle;
+
+	my $msg = "In file included";
+	printf "%s from %s,\n", $msg, $last->[1] if defined $last;
+
+	for my $header (reverse @$cycle) {
+		printf "%s from %s:%d%s\n",
+			" " x length $msg,
+			$header->[1], $header->[0],
+			$header->[1] eq $last->[1] ? ' <-- here' : '';
+	}
+
+	printf "%s:%d: warning: recursive header inclusion\n",
+		$first->[1], $first->[0];
+}
+
+# Find and print the smallest cycle starting in the specified node.
+sub detect_cycles {
+	my @queue = map { [[0, $_]] } @_;
+	while(@queue) {
+		my $top = pop @queue;
+		my $name = $top->[-1]->[1];
+
+		for my $dep (@{$deps{$name}}) {
+			my $chain = [@$top, [$dep->[0], $dep->[1]]];
+
+			# If the dep already exists in the chain, we have a
+			# cycle...
+			if(grep { $_->[1] eq $dep->[1] } @$top) {
+				print_cycle($chain);
+				next if $opt_all;
+				return;
+			}
+
+			push @queue, $chain;
+		}
+	}
+}
+
+sub mangle {
+	$_ = shift;
+	s/\//__/g;
+	s/\./_/g;
+	s/-/_/g;
+	$_;
+}
+
+# Output dependency graph in GraphViz language.
+sub graph {
+	print "digraph {\n";
+
+	print "\t/* vertices */\n";
+	for my $header (keys %deps) {
+		printf "\t%s [label=\"%s\"];\n",
+			mangle($header), $header;
+	}
+
+	print "\n";
+
+	print "\t/* edges */\n";
+	for my $header (keys %deps) {
+		for my $dep (@{$deps{$header}}) {
+			printf "\t%s -> %s;\n",
+				mangle($header), mangle($dep->[1]);
+		}
+	}
+
+	print "}\n";
+}