rtla: Add rtla timerlat top documentation

Man page for rtla timerlat top mode.

Link: https://lkml.kernel.org/r/58c3d6212e6c6f1f012deb2e998dd082da92075f.1639158831.git.bristot@kernel.org

Cc: Tao Zhou <tao.zhou@linux.dev>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Tom Zanussi <zanussi@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Clark Williams <williams@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
Cc: linux-rt-users@vger.kernel.org
Cc: linux-trace-devel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Documentation/tools/rtla/common_timerlat_options.rst

+**-p**, **--period** *us*
+
+        Set the *timerlat* tracer period in microseconds.
+
+**-i**, **--irq** *us*
+
+        Stop trace if the *IRQ* latency is higher than the argument in us.
+
+**-T**, **--thread** *us*
+
+        Stop trace if the *Thread* latency is higher than the argument in us.
+
+**-s**, **--stack** *us*
+
+        Save the stack trace at the *IRQ* if a *Thread* latency is higher than the
+        argument in us.

Documentation/tools/rtla/rtla-timerlat-top.rst

+====================
+rtla-timerlat-top
+====================
+-------------------------------------------
+Measures the operating system timer latency
+-------------------------------------------
+
+:Manual section: 1
+
+SYNOPSIS
+========
+**rtla timerlat top** [*OPTIONS*] ...
+
+DESCRIPTION
+===========
+
+.. include:: common_timerlat_description.rst
+
+The **rtla timerlat top** displays a summary of the periodic output
+from the *timerlat* tracer. It also provides information for each
+operating system noise via the **osnoise:** tracepoints that can be
+seem with the option **-T**.
+
+OPTIONS
+=======
+
+.. include:: common_timerlat_options.rst
+
+.. include:: common_top_options.rst
+
+.. include:: common_options.rst
+
+EXAMPLE
+=======
+
+In the example below, the *timerlat* tracer is set to capture the stack trace at
+the IRQ handler, printing it to the buffer if the *Thread* timer latency is
+higher than *30 us*. It is also set to stop the session if a *Thread* timer
+latency higher than *30 us* is hit. Finally, it is set to save the trace
+buffer if the stop condition is hit::
+
+  [root@alien ~]# rtla timerlat top -s 30 -t 30 -T
+                   Timer Latency
+    0 00:00:59   |          IRQ Timer Latency (us)        |         Thread Timer Latency (us)
+  CPU COUNT      |      cur       min       avg       max |      cur       min       avg       max
+    0 #58634     |        1         0         1        10 |       11         2        10        23
+    1 #58634     |        1         0         1         9 |       12         2         9        23
+    2 #58634     |        0         0         1        11 |       10         2         9        23
+    3 #58634     |        1         0         1        11 |       11         2         9        24
+    4 #58634     |        1         0         1        10 |       11         2         9        26
+    5 #58634     |        1         0         1         8 |       10         2         9        25
+    6 #58634     |       12         0         1        12 |       30         2        10        30 <--- CPU with spike
+    7 #58634     |        1         0         1         9 |       11         2         9        23
+    8 #58633     |        1         0         1         9 |       11         2         9        26
+    9 #58633     |        1         0         1         9 |       10         2         9        26
+   10 #58633     |        1         0         1        13 |       11         2         9        28
+   11 #58633     |        1         0         1        13 |       12         2         9        24
+   12 #58633     |        1         0         1         8 |       10         2         9        23
+   13 #58633     |        1         0         1        10 |       10         2         9        22
+   14 #58633     |        1         0         1        18 |       12         2         9        27
+   15 #58633     |        1         0         1        10 |       11         2         9        28
+   16 #58633     |        0         0         1        11 |        7         2         9        26
+   17 #58633     |        1         0         1        13 |       10         2         9        24
+   18 #58633     |        1         0         1         9 |       13         2         9        22
+   19 #58633     |        1         0         1        10 |       11         2         9        23
+   20 #58633     |        1         0         1        12 |       11         2         9        28
+   21 #58633     |        1         0         1        14 |       11         2         9        24
+   22 #58633     |        1         0         1         8 |       11         2         9        22
+   23 #58633     |        1         0         1        10 |       11         2         9        27
+  timerlat hit stop tracing
+  saving trace to timerlat_trace.txt
+  [root@alien bristot]# tail -60 timerlat_trace.txt
+  [...]
+      timerlat/5-79755   [005] .......   426.271226: #58634 context thread timer_latency     10823 ns
+              sh-109404  [006] dnLh213   426.271247: #58634 context    irq timer_latency     12505 ns
+              sh-109404  [006] dNLh313   426.271258: irq_noise: local_timer:236 start 426.271245463 duration 12553 ns
+              sh-109404  [006] d...313   426.271263: thread_noise:       sh:109404 start 426.271245853 duration 4769 ns
+      timerlat/6-79756   [006] .......   426.271264: #58634 context thread timer_latency     30328 ns
+      timerlat/6-79756   [006] ....1..   426.271265: <stack trace>
+  => timerlat_irq
+  => __hrtimer_run_queues
+  => hrtimer_interrupt
+  => __sysvec_apic_timer_interrupt
+  => sysvec_apic_timer_interrupt
+  => asm_sysvec_apic_timer_interrupt
+  => _raw_spin_unlock_irqrestore			<---- spinlock that disabled interrupt.
+  => try_to_wake_up
+  => autoremove_wake_function
+  => __wake_up_common
+  => __wake_up_common_lock
+  => ep_poll_callback
+  => __wake_up_common
+  => __wake_up_common_lock
+  => fsnotify_add_event
+  => inotify_handle_inode_event
+  => fsnotify
+  => __fsnotify_parent
+  => __fput
+  => task_work_run
+  => exit_to_user_mode_prepare
+  => syscall_exit_to_user_mode
+  => do_syscall_64
+  => entry_SYSCALL_64_after_hwframe
+  => 0x7265000001378c
+  => 0x10000cea7
+  => 0x25a00000204a
+  => 0x12e302d00000000
+  => 0x19b51010901b6
+  => 0x283ce00726500
+  => 0x61ea308872
+  => 0x00000fe3
+            bash-109109  [007] d..h...   426.271265: #58634 context    irq timer_latency      1211 ns
+      timerlat/6-79756   [006] .......   426.271267: timerlat_main: stop tracing hit on cpu 6
+
+In the trace, it is possible the notice that the *IRQ* timer latency was
+already high, accounting *12505 ns*. The IRQ delay was caused by the
+*bash-109109* process that disabled IRQs in the wake-up path
+(*_try_to_wake_up()* function). The duration of the IRQ handler that woke
+up the timerlat thread, informed with the **osnoise:irq_noise** event, was
+also high and added more *12553 ns* to the Thread latency. Finally, the
+**osnoise:thread_noise** added by the currently running thread (including
+the scheduling overhead) added more *4769 ns*. Summing up these values,
+the *Thread* timer latency accounted for *30328 ns*.
+
+The primary reason for this high value is the wake-up path that was hit
+twice during this case: when the *bash-109109* was waking up a thread
+and then when the *timerlat* thread was awakened. This information can
+then be used as the starting point of a more fine-grained analysis.
+
+Note that **rtla timerlat** was dispatched without changing *timerlat* tracer
+threads' priority. That is generally not needed because these threads hava
+priority *FIFO:95* by default, which is a common priority used by real-time
+kernel developers to analyze scheduling delays.
+
+SEE ALSO
+--------
+**rtla-timerlat**\(1), **rtla-timerlat-hist**\(1)
+
+*timerlat* tracer documentation: <https://www.kernel.org/doc/html/latest/trace/timerlat-tracer.html>
+
+AUTHOR
+------
+Written by Daniel Bristot de Oliveira <bristot@kernel.org>
+
+.. include:: common_appendix.rst