tools/rcu: Add rcu-updaters.sh script

This commit adds a tools/rcu/rcu-updaters.sh script that uses bpftrace
to print a histogram of the RCU update-side primitives invoked during
the specified time interval, or until manually terminated if no interval
is specified.

Sample output on an idle laptop:

@counts[poll_state_synchronize_rcu]: 6
@counts[synchronize_srcu]: 13
@counts[call_rcu_tasks_trace]: 25
@counts[synchronize_rcu]: 54
@counts[kvfree_call_rcu]: 428
@counts[call_rcu]: 2134

Note that when run on a kernel missing one or more of the symbols, this
script will issue a diagnostic for each that is not found, but continue
normally for the rest of the functions.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/rcu/rcu-updaters.sh

+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0+
+#
+# Run bpftrace to obtain a histogram of the types of primitives used to
+# initiate RCU grace periods.  The count associated with rcu_gp_init()
+# is the number of normal (non-expedited) grace periods.
+#
+# Usage: rcu-updaters.sh [ duration-in-seconds ]
+#
+# Note that not all kernel builds have all of these functions.  In those
+# that do not, this script will issue a diagnostic for each that is not
+# found, but continue normally for the rest of the functions.
+
+duration=${1}
+if test -n "${duration}"
+then
+	exitclause='interval:s:'"${duration}"' { exit(); }'
+else
+	echo 'Hit control-C to end sample and print results.'
+fi
+bpftrace -e 'kprobe:kvfree_call_rcu,
+	     kprobe:call_rcu,
+	     kprobe:call_rcu_tasks,
+	     kprobe:call_rcu_tasks_rude,
+	     kprobe:call_rcu_tasks_trace,
+	     kprobe:call_srcu,
+	     kprobe:rcu_barrier,
+	     kprobe:rcu_barrier_tasks,
+	     kprobe:rcu_barrier_tasks_rude,
+	     kprobe:rcu_barrier_tasks_trace,
+	     kprobe:srcu_barrier,
+	     kprobe:synchronize_rcu,
+	     kprobe:synchronize_rcu_expedited,
+	     kprobe:synchronize_rcu_tasks,
+	     kprobe:synchronize_rcu_tasks_rude,
+	     kprobe:synchronize_rcu_tasks_trace,
+	     kprobe:synchronize_srcu,
+	     kprobe:synchronize_srcu_expedited,
+	     kprobe:get_state_synchronize_rcu,
+	     kprobe:get_state_synchronize_rcu_full,
+	     kprobe:start_poll_synchronize_rcu,
+	     kprobe:start_poll_synchronize_rcu_expedited,
+	     kprobe:start_poll_synchronize_rcu_full,
+	     kprobe:start_poll_synchronize_rcu_expedited_full,
+	     kprobe:poll_state_synchronize_rcu,
+	     kprobe:poll_state_synchronize_rcu_full,
+	     kprobe:cond_synchronize_rcu,
+	     kprobe:cond_synchronize_rcu_full,
+	     kprobe:start_poll_synchronize_srcu,
+	     kprobe:poll_state_synchronize_srcu,
+	     kprobe:rcu_gp_init
+	     	{ @counts[func] = count(); } '"${exitclause}"