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, 60 insertions, 0 deletions

diff --git a/scripts/stackdelta b/scripts/stackdelta
new file mode 100755
index 000000000..44d2dfd62
--- /dev/null
+++ b/scripts/stackdelta
@@ -0,0 +1,60 @@
+#!/usr/bin/env perl
+# SPDX-License-Identifier: GPL-2.0
+
+# Read two files produced by the stackusage script, and show the
+# delta between them.
+#
+# Currently, only shows changes for functions listed in both files. We
+# could add an option to show also functions which have vanished or
+# appeared (which would often be due to gcc making other inlining
+# decisions).
+#
+# Another possible option would be a minimum absolute value for the
+# delta.
+#
+# A third possibility is for sorting by delta, but that can be
+# achieved by piping to sort -k5,5g.
+
+sub read_stack_usage_file {
+    my %su;
+    my $f = shift;
+    open(my $fh, '<', $f)
+	or die "cannot open $f: $!";
+    while (<$fh>) {
+	chomp;
+	my ($file, $func, $size, $type) = split;
+	# Old versions of gcc (at least 4.7) have an annoying quirk in
+	# that a (static) function whose name has been changed into
+	# for example ext4_find_unwritten_pgoff.isra.11 will show up
+	# in the .su file with a name of just "11". Since such a
+	# numeric suffix is likely to change across different
+	# commits/compilers/.configs or whatever else we're trying to
+	# tweak, we can't really track those functions, so we just
+	# silently skip them.
+	#
+	# Newer gcc (at least 5.0) report the full name, so again,
+	# since the suffix is likely to change, we strip it.
+	next if $func =~ m/^[0-9]+$/;
+	$func =~ s/\..*$//;
+	# Line numbers are likely to change; strip those.
+	$file =~ s/:[0-9]+$//;
+	$su{"${file}\t${func}"} = {size => $size, type => $type};
+    }
+    close($fh);
+    return \%su;
+}
+
+@ARGV == 2
+    or die "usage: $0 <old> <new>";
+
+my $old = read_stack_usage_file($ARGV[0]);
+my $new = read_stack_usage_file($ARGV[1]);
+my @common = sort grep {exists $new->{$_}} keys %$old;
+for (@common) {
+    my $x = $old->{$_}{size};
+    my $y = $new->{$_}{size};
+    my $delta = $y - $x;
+    if ($delta) {
+	printf "%s\t%d\t%d\t%+d\n", $_, $x, $y, $delta;
+    }
+}