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, 134 insertions, 0 deletions

diff --git a/scripts/tracing/ftrace-bisect.sh b/scripts/tracing/ftrace-bisect.sh
new file mode 100755
index 000000000..bb4f59262
--- /dev/null
+++ b/scripts/tracing/ftrace-bisect.sh
@@ -0,0 +1,134 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Here's how to use this:
+#
+# This script is used to help find functions that are being traced by function
+# tracer or function graph tracing that causes the machine to reboot, hang, or
+# crash. Here's the steps to take.
+#
+# First, determine if function tracing is working with a single function:
+#
+#   (note, if this is a problem with function_graph tracing, then simply
+#    replace "function" with "function_graph" in the following steps).
+#
+#  # cd /sys/kernel/tracing
+#  # echo schedule > set_ftrace_filter
+#  # echo function > current_tracer
+#
+# If this works, then we know that something is being traced that shouldn't be.
+#
+#  # echo nop > current_tracer
+#
+# Starting with v5.1 this can be done with numbers, making it much faster:
+#
+# The old (slow) way, for kernels before v5.1.
+#
+# [old-way] # cat available_filter_functions > ~/full-file
+#
+# [old-way] *** Note ***  this process will take several minutes to update the
+# [old-way] filters. Setting multiple functions is an O(n^2) operation, and we
+# [old-way] are dealing with thousands of functions. So go have coffee, talk
+# [old-way] with your coworkers, read facebook. And eventually, this operation
+# [old-way] will end.
+#
+# The new way (using numbers) is an O(n) operation, and usually takes less than a second.
+#
+# seq `wc -l available_filter_functions | cut -d' ' -f1` > ~/full-file
+#
+# This will create a sequence of numbers that match the functions in
+# available_filter_functions, and when echoing in a number into the
+# set_ftrace_filter file, it will enable the corresponding function in
+# O(1) time. Making enabling all functions O(n) where n is the number of
+# functions to enable.
+#
+# For either the new or old way, the rest of the operations remain the same.
+#
+#  # ftrace-bisect ~/full-file ~/test-file ~/non-test-file
+#  # cat ~/test-file > set_ftrace_filter
+#
+#  # echo function > current_tracer
+#
+# If it crashes, we know that ~/test-file has a bad function.
+#
+#   Reboot back to test kernel.
+#
+#     # cd /sys/kernel/tracing
+#     # mv ~/test-file ~/full-file
+#
+# If it didn't crash.
+#
+#     # echo nop > current_tracer
+#     # mv ~/non-test-file ~/full-file
+#
+# Get rid of the other test file from previous run (or save them off somewhere).
+#  # rm -f ~/test-file ~/non-test-file
+#
+# And start again:
+#
+#  # ftrace-bisect ~/full-file ~/test-file ~/non-test-file
+#
+# The good thing is, because this cuts the number of functions in ~/test-file
+# by half, the cat of it into set_ftrace_filter takes half as long each
+# iteration, so don't talk so much at the water cooler the second time.
+#
+# Eventually, if you did this correctly, you will get down to the problem
+# function, and all we need to do is to notrace it.
+#
+# The way to figure out if the problem function is bad, just do:
+#
+#  # echo <problem-function> > set_ftrace_notrace
+#  # echo > set_ftrace_filter
+#  # echo function > current_tracer
+#
+# And if it doesn't crash, we are done.
+#
+# If it does crash, do this again (there's more than one problem function)
+# but you need to echo the problem function(s) into set_ftrace_notrace before
+# enabling function tracing in the above steps. Or if you can compile the
+# kernel, annotate the problem functions with "notrace" and start again.
+#
+
+
+if [ $# -ne 3 ]; then
+  echo 'usage: ftrace-bisect full-file test-file  non-test-file'
+  exit
+fi
+
+full=$1
+test=$2
+nontest=$3
+
+x=`cat $full | wc -l`
+if [ $x -eq 1 ]; then
+	echo "There's only one function left, must be the bad one"
+	cat $full
+	exit 0
+fi
+
+let x=$x/2
+let y=$x+1
+
+if [ ! -f $full ]; then
+	echo "$full does not exist"
+	exit 1
+fi
+
+if [ -f $test ]; then
+	echo -n "$test exists, delete it? [y/N]"
+	read a
+	if [ "$a" != "y" -a "$a" != "Y" ]; then
+		exit 1
+	fi
+fi
+
+if [ -f $nontest ]; then
+	echo -n "$nontest exists, delete it? [y/N]"
+	read a
+	if [ "$a" != "y" -a "$a" != "Y" ]; then
+		exit 1
+	fi
+fi
+
+sed -ne "1,${x}p" $full > $test
+sed -ne "$y,\$p" $full > $nontest