hwlat improvements and osnoise/timerlat tracers

This series proposes a set of improvements and new features for the tracing subsystem to facilitate the debugging of low latency deployments.

Currently, hwlat runs on a single CPU at a time, migrating across a set of CPUs in a round-robin fashion. This series improves hwlat to allow hwlat to run on multiple CPUs in parallel, increasing the chances of detecting a hardware latency, at the cost of using more CPU time.

It also proposes a new tracer named osnoise, that aims to help users of isolcpus= (or a similar method) to measure how much noise the OS and the hardware add to the isolated application. The osnoise tracer bases on the hwlat detector code. The difference is that, instead of sampling with interrupts disabled, the osnoise tracer samples the CPU with interrupts and preemption enabled. In this way, the sampling thread will suffer any source of noise from the OS. The detection and classification of the type of noise are then made by observing the entry points of NMIs, IRQs, SoftIRQs, and threads. If none of these sources of noise is detected, the tool associates the noise with the hardware. The tool periodically prints a status, printing the total noise of the period, the max single noise observed, the percentage of CPU available for the task, along with the counters of each source of the noise. To debug the sources of noise, the tracer also adds a set of tracepoints that print any NMI, IRQ, SofIRQ, and thread occurrence. These tracepoints print the starting time and the noise's net duration at the end of the noise. In this way, it reduces the number of tracepoints (one instead of two) and the need to manually accounting the contribution of each noise independently.

Finaly, the timerlat tracer aims to help the preemptive kernel developers to find sources of wakeup latencies of real-time threads. The tracer creates a per-cpu kernel thread with real-time priority. The tracer thread sets a periodic timer to wakeup itself, and goes to sleep waiting for the timer to fire. At the wakeup, the thread then computes a wakeup latency value as the difference between the current time and the absolute time that the timer was set to expire. The tracer prints two lines at every activation. The first is the timer latency observed at the hardirq context before the activation of the thread. The second is the timer latency observed by the thread, which is the same level that cyclictest reports. The ACTIVATION ID field serves to relate the irq execution to its respective thread execution. The tracer is build on top of osnoise tracer, and the osnoise: events can be used to trace the source of interference from NMI, IRQs and other threads. It also enables the capture of the stacktrace at the IRQ context, which helps to identify the code path that can cause thread delay.

Changes from v4:
- If tracing_threshold is set to 0, use 5 us as the osnoise threshold - instead of the 1 us previously used. (Bristot)
- Fix references to stop_tracing_us/stop_tracing_total_us on timerlat documentation (Bristot).
- Raname ULL_STR_SIZE to U64_STR_SIZE (Bristot)
- Make ULL_STR_SIZE 24 (Rostedt)
- Fix ifdefs on arch/x86/kernel/trace.c (Rostedt)
- Remove unneeded kernel/trace/trace_osnoise.h (Rostedt)
- Fix hotplug locking issues (Rostedt)
- Fix <spaces> on places of <tab> (Rostedt)

Changes from v3:
- Remove /** from comments (Rostedt)
- Support hotplug operations (Rostedt)
- Change the name of ull_config to trace_min_max - and improve patch description (Rostedt)
- Remove leftovers from older versions (Juri Lelli)
- Remove printk from main loop - use trace buffer instead (Rostedt)
- Move the arch specific code to arch/*/kernel/trace.c (Rostedt)
- Improve timerlat documentation (Rostedt)
- Add () around params usage of "value" on __print_ns_to_secs(value) (Rostedt)
- Rename osnoise/stop_tracing* files (Rostedt)
- Reduce the size of the per-cpu data to store stack tracers on timerlat (Rostedt)
- Add osn_* prefix to structures (Rostedt)
- Fix stop tracing to work with trace instances (Rostedt)
- Changing osnoise/cpus restarts the threads accordingly (Rostedt)
- Lots of cosmetics/typos changes (Rostedt)

Changes from v2:
- osnoise sample reports in nanoseconds (as all other osnoise tracepoints) (Bristot)
- Remove divisions from osnoise main loop (Bristot)
- Make the tracers work well when starting via kernel-cmdline (Red Hat's performance team need)
- Rename main/interrupt functions (Bristot)
- Fix timerlat reset (Juri Lelli)
- Fix timerlat start (Juri Lelli)

Changes from v1:
- Remove `` from RST (Corbet)
- Add RST files to the index (Corbet)
- Fix text and typos (Rostedt)
- Remove the cpus from hwlat (Rostedt)
- Remove the disable_migrate/fallback to mode none on hwlat (Rostedt)
- Add a generic way to read/write u64 and use it on hwlat/osnoise/timerlat (Rostedt)
- Make osnoise/timerlat to work properly with trace-cmd/tracer instances (Rostedt)
- osnoise using the tracing_threshold (Rostedt)
- Rearrange tracepoint structure to avoid "holes" (Rostedt)

Daniel Bristot de Oliveira (13):
trace/hwlat: Fix Clark's email
trace/hwlat: Implement the mode config option trace/hwlat: Switch disable_migrate to mode none trace/hwlat: Implement the per-cpu mode
trace: Add a generic function to read/write u64 values from tracefs trace/hwlat: Use trace_min_max_param for width and window params trace/hwlat: Remove printk from sampling loop
trace: Add osnoise tracer
trace: Add timerlat tracer
trace/hwlat: Protect kdata->kthread with get/put_online_cpus
trace: Protect tr->tracing_cpumask with get/put_online_cpus trace/hwlat: Support hotplug operations
trace/osnoise: Support hotplug operations

Steven Rostedt (1):
trace: Add __print_ns_to_secs() and __print_ns_without_secs() helpers

 Documentation/trace/hwlat_detector.rst  |   13 +-
 Documentation/trace/index.rst           |    2 +
 Documentation/trace/osnoise-tracer.rst  |  152 ++
 Documentation/trace/timerlat-tracer.rst |  181 ++
 arch/x86/kernel/Makefile                |    1 +
 arch/x86/kernel/trace.c                 |  236 +++
 include/linux/ftrace_irq.h              |   13 +
 include/trace/events/osnoise.h          |  142 ++
 include/trace/trace_events.h            |   25 +
 kernel/trace/Kconfig                    |   62 +
 kernel/trace/Makefile                   |    1 +
 kernel/trace/trace.c                    |   91 +
 kernel/trace/trace.h                    |   29 +-
 kernel/trace/trace_entries.h            |   41 +
 kernel/trace/trace_hwlat.c              |  534 ++++--
 kernel/trace/trace_osnoise.c            | 2051 +++++++++++++++++++++++
 kernel/trace/trace_output.c             |  119 +-
 17 files changed, 3543 insertions(+), 150 deletions(-)
 create mode 100644 Documentation/trace/osnoise-tracer.rst
 create mode 100644 Documentation/trace/timerlat-tracer.rst
 create mode 100644 arch/x86/kernel/trace.c
 create mode 100644 include/trace/events/osnoise.h
 create mode 100644 kernel/trace/trace_osnoise.c

2.31.1