Documentation/rtla: Add hwnoise man page

Add a man page for the new rtla hwnoise tool, mostly based in the
rtla osnoise top.

Link: https://lkml.kernel.org/r/27088e0f42a6129e9b75f789d78adff4171f2e79.1675805361.git.bristot@kernel.org

Signed-off-by: Daniel Bristot de Oliveira <[email protected]>
Reviewed-by: Bagas Sanjaya <[email protected]>
Cc: Daniel Bristot de Oliveira <[email protected]>
Cc: Juri Lelli <[email protected]>
Cc: Clark Williams <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: Daniel Bristot de Oliveira

Documentation/tools/rtla/index.rst

@@ -17,6 +17,7 @@ behavior on specific hardware.
    rtla-timerlat
    rtla-timerlat-hist
    rtla-timerlat-top
+   rtla-hwnoise
 
 .. only::  subproject and html
 

Documentation/tools/rtla/rtla-hwnoise.rst

@@ -0,0 +1,107 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+============
+rtla-hwnoise
+============
+------------------------------------------
+Detect and quantify hardware-related noise
+------------------------------------------
+
+:Manual section: 1
+
+SYNOPSIS
+========
+
+**rtla hwnoise** [*OPTIONS*]
+
+DESCRIPTION
+===========
+
+**rtla hwnoise** collects the periodic summary from the *osnoise* tracer
+running with *interrupts disabled*. By disabling interrupts, and the scheduling
+of threads as a consequence, only non-maskable interrupts and hardware-related
+noise is allowed.
+
+The tool also allows the configurations of the *osnoise* tracer and the
+collection of the tracer output.
+
+OPTIONS
+=======
+.. include:: common_osnoise_options.rst
+
+.. include:: common_top_options.rst
+
+.. include:: common_options.rst
+
+EXAMPLE
+=======
+In the example below, the **rtla hwnoise** tool is set to run on CPUs *1-7*
+on a system with 8 cores/16 threads with hyper-threading enabled.
+
+The tool is set to detect any noise higher than *one microsecond*,
+to run for *ten minutes*, displaying a summary of the report at the
+end of the session::
+
+  # rtla hwnoise -c 1-7 -T 1 -d 10m -q
+                                          Hardware-related Noise
+  duration:   0 00:10:00 | time is in us
+  CPU Period       Runtime        Noise  % CPU Aval   Max Noise   Max Single          HW          NMI
+    1 #599       599000000          138    99.99997           3            3           4           74
+    2 #599       599000000           85    99.99998           3            3           4           75
+    3 #599       599000000           86    99.99998           4            3           6           75
+    4 #599       599000000           81    99.99998           4            4           2           75
+    5 #599       599000000           85    99.99998           2            2           2           75
+    6 #599       599000000           76    99.99998           2            2           0           75
+    7 #599       599000000           77    99.99998           3            3           0           75
+
+
+The first column shows the *CPU*, and the second column shows how many
+*Periods* the tool ran during the session. The *Runtime* is the time
+the tool effectively runs on the CPU. The *Noise* column is the sum of
+all noise that the tool observed, and the *% CPU Aval* is the relation
+between the *Runtime* and *Noise*.
+
+The *Max Noise* column is the maximum hardware noise the tool detected in a
+single period, and the *Max Single* is the maximum single noise seen.
+
+The *HW* and *NMI* columns show the total number of *hardware* and *NMI* noise
+occurrence observed by the tool.
+
+For example, *CPU 3* ran *599* periods of *1 second Runtime*. The CPU received
+*86 us* of noise during the entire execution, leaving *99.99997 %* of CPU time
+for the application. In the worst single period, the CPU caused *4 us* of
+noise to the application, but it was certainly caused by more than one single
+noise, as the *Max Single* noise was of *3 us*. The CPU has *HW noise,* at a
+rate of *six occurrences*/*ten minutes*. The CPU also has *NMIs*, at a higher
+frequency: around *seven per second*.
+
+The tool should report *0* hardware-related noise in the ideal situation.
+For example, by disabling hyper-threading to remove the hardware noise,
+and disabling the TSC watchdog to remove the NMI (it is possible to identify
+this using tracing options of **rtla hwnoise**), it was possible to reach
+the ideal situation in the same hardware::
+
+  # rtla hwnoise -c 1-7 -T 1 -d 10m -q
+                                          Hardware-related Noise
+  duration:   0 00:10:00 | time is in us
+  CPU Period       Runtime        Noise  % CPU Aval   Max Noise   Max Single          HW          NMI
+    1 #599       599000000            0   100.00000           0            0           0            0
+    2 #599       599000000            0   100.00000           0            0           0            0
+    3 #599       599000000            0   100.00000           0            0           0            0
+    4 #599       599000000            0   100.00000           0            0           0            0
+    5 #599       599000000            0   100.00000           0            0           0            0
+    6 #599       599000000            0   100.00000           0            0           0            0
+    7 #599       599000000            0   100.00000           0            0           0            0
+
+SEE ALSO
+========
+
+**rtla-osnoise**\(1)
+
+Osnoise tracer documentation: <https://www.kernel.org/doc/html/latest/trace/osnoise-tracer.html>
+
+AUTHOR
+======
+Written by Daniel Bristot de Oliveira <[email protected]>
+
+.. include:: common_appendix.rst