tools/rtla: Add -U/--user-load option to timerlat

The timerlat tracer provides an interface for any application to wait
for the timerlat's periodic wakeup. Currently, rtla timerlat uses it
to dispatch its user-space workload (-u option).

But as the tracer interface is generic, rtla timerlat can also be used
to monitor any workload that uses it. For example, a user might
place their own workload to wait on the tracer interface, and
monitor the results with rtla timerlat.

Add the -U option to rtla timerlat top and hist. With this option, rtla
timerlat will not dispatch its workload but only setting up the
system, waiting for a user to dispatch its workload.

The sample code in this patch is an example of python application
that loops in the timerlat tracer fd.

To use it, dispatch:

 # rtla timerlat -U

In a terminal, then run the python program on another terminal,
specifying the CPU to run it. For example, setting on CPU 1:

 #./timerlat_load.py 1

Then rtla timerlat will start printing the statistics of the
./timerlat_load.py app.

An interesting point is that the "Ret user Timer Latency" value
is the overall response time of the load. The sample load does
a memory copy to exemplify that.

The stop tracing options on rtla timerlat works in this setup
as well, including auto analysis.

Link: https://lkml.kernel.org/r/36e6bcf18fe15c7601048fd4c65aeb193c502cc8.1707229706.git.bristot@kernel.org

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>

tools/tracing/rtla/sample/timerlat_load.py

@@ -0,0 +1,74 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# Copyright (C) 2024 Red Hat, Inc. Daniel Bristot de Oliveira <bristot@kernel.org>
+#
+# This is a sample code about how to use timerlat's timer by any workload
+# so rtla can measure and provide auto-analysis for the overall latency (IOW
+# the response time) for a task.
+#
+# Before running it, you need to dispatch timerlat with -U option in a terminal.
+# Then # run this script pinned to a CPU on another terminal. For example:
+#
+# timerlat_load.py 1 -p 95
+#
+# The "Timerlat IRQ" is the IRQ latency, The thread latency is the latency
+# for the python process to get the CPU. The Ret from user Timer Latency is
+# the overall latency. In other words, it is the response time for that
+# activation.
+#
+# This is just an example, the load is reading 20MB of data from /dev/full
+# It is in python because it is easy to read :-)
+
+import argparse
+import sys
+import os
+
+parser = argparse.ArgumentParser(description='user-space timerlat thread in Python')
+parser.add_argument("cpu", help='CPU to run timerlat thread')
+parser.add_argument("-p", "--prio", help='FIFO priority')
+
+args = parser.parse_args()
+
+try:
+    affinity_mask = { int(args.cpu) }
+except:
+    print("Invalid cpu: " + args.cpu)
+    exit(1)
+
+try:
+    os.sched_setaffinity(0, affinity_mask);
+except:
+    print("Error setting affinity")
+    exit(1)
+
+if (args.prio):
+    try:
+        param = os.sched_param(int(args.prio))
+        os.sched_setscheduler(0, os.SCHED_FIFO, param)
+    except:
+        print("Error setting priority")
+        exit(1)
+
+try:
+    timerlat_path = "/sys/kernel/tracing/osnoise/per_cpu/cpu" + args.cpu + "/timerlat_fd"
+    timerlat_fd = open(timerlat_path, 'r')
+except:
+    print("Error opening timerlat fd, did you run timerlat -U?")
+    exit(1)
+
+try:
+    data_fd = open("/dev/full", 'r');
+except:
+    print("Error opening data fd")
+
+while True:
+    try:
+        timerlat_fd.read(1)
+        data_fd.read(20*1024*1024)
+    except:
+        print("Leaving")
+        break
+
+timerlat_fd.close()
+data_fd.close()