1==============
2OSNOISE Tracer
3==============
4
5In the context of high-performance computing (HPC), the Operating System
6Noise (*osnoise*) refers to the interference experienced by an application
7due to activities inside the operating system. In the context of Linux,
8NMIs, IRQs, SoftIRQs, and any other system thread can cause noise to the
9system. Moreover, hardware-related jobs can also cause noise, for example,
10via SMIs.
11
12hwlat_detector is one of the tools used to identify the most complex
13source of noise: *hardware noise*.
14
15In a nutshell, the hwlat_detector creates a thread that runs
16periodically for a given period. At the beginning of a period, the thread
17disables interrupt and starts sampling. While running, the hwlatd
18thread reads the time in a loop. As interrupts are disabled, threads,
19IRQs, and SoftIRQs cannot interfere with the hwlatd thread. Hence, the
20cause of any gap between two different reads of the time roots either on
21NMI or in the hardware itself. At the end of the period, hwlatd enables
22interrupts and reports the max observed gap between the reads. It also
23prints a NMI occurrence counter. If the output does not report NMI
24executions, the user can conclude that the hardware is the culprit for
25the latency. The hwlat detects the NMI execution by observing
26the entry and exit of a NMI.
27
28The osnoise tracer leverages the hwlat_detector by running a
29similar loop with preemption, SoftIRQs and IRQs enabled, thus allowing
30all the sources of *osnoise* during its execution. Using the same approach
31of hwlat, osnoise takes note of the entry and exit point of any
32source of interferences, increasing a per-cpu interference counter. The
33osnoise tracer also saves an interference counter for each source of
34interference. The interference counter for NMI, IRQs, SoftIRQs, and
35threads is increased anytime the tool observes these interferences' entry
36events. When a noise happens without any interference from the operating
37system level, the hardware noise counter increases, pointing to a
38hardware-related noise. In this way, osnoise can account for any
39source of interference. At the end of the period, the osnoise tracer
40prints the sum of all noise, the max single noise, the percentage of CPU
41available for the thread, and the counters for the noise sources.
42
43Usage
44-----
45
46Write the ASCII text "osnoise" into the current_tracer file of the
47tracing system (generally mounted at /sys/kernel/tracing).
48
49For example::
50
51        [root@f32 ~]# cd /sys/kernel/tracing/
52        [root@f32 tracing]# echo osnoise > current_tracer
53
54It is possible to follow the trace by reading the trace file::
55
56        [root@f32 tracing]# cat trace
57        # tracer: osnoise
58        #
59        #                                _-----=> irqs-off
60        #                               / _----=> need-resched
61        #                              | / _---=> hardirq/softirq
62        #                              || / _--=> preempt-depth                            MAX
63        #                              || /                                             SINGLE     Interference counters:
64        #                              ||||               RUNTIME      NOISE   % OF CPU  NOISE    +-----------------------------+
65        #           TASK-PID      CPU# ||||   TIMESTAMP    IN US       IN US  AVAILABLE  IN US     HW    NMI    IRQ   SIRQ THREAD
66        #              | |         |   ||||      |           |             |    |            |      |      |      |      |      |
67                   <...>-859     [000] ....    81.637220: 1000000        190  99.98100       9     18      0   1007     18      1
68                   <...>-860     [001] ....    81.638154: 1000000        656  99.93440      74     23      0   1006     16      3
69                   <...>-861     [002] ....    81.638193: 1000000       5675  99.43250     202      6      0   1013     25     21
70                   <...>-862     [003] ....    81.638242: 1000000        125  99.98750      45      1      0   1011     23      0
71                   <...>-863     [004] ....    81.638260: 1000000       1721  99.82790     168      7      0   1002     49     41
72                   <...>-864     [005] ....    81.638286: 1000000        263  99.97370      57      6      0   1006     26      2
73                   <...>-865     [006] ....    81.638302: 1000000        109  99.98910      21      3      0   1006     18      1
74                   <...>-866     [007] ....    81.638326: 1000000       7816  99.21840     107      8      0   1016     39     19
75
76In addition to the regular trace fields (from TASK-PID to TIMESTAMP), the
77tracer prints a message at the end of each period for each CPU that is
78running an osnoise/ thread. The osnoise specific fields report:
79
80 - The RUNTIME IN US reports the amount of time in microseconds that
81   the osnoise thread kept looping reading the time.
82 - The NOISE IN US reports the sum of noise in microseconds observed
83   by the osnoise tracer during the associated runtime.
84 - The % OF CPU AVAILABLE reports the percentage of CPU available for
85   the osnoise thread during the runtime window.
86 - The MAX SINGLE NOISE IN US reports the maximum single noise observed
87   during the runtime window.
88 - The Interference counters display how many each of the respective
89   interference happened during the runtime window.
90
91Note that the example above shows a high number of HW noise samples.
92The reason being is that this sample was taken on a virtual machine,
93and the host interference is detected as a hardware interference.
94
95Tracer options
96---------------------
97
98The tracer has a set of options inside the osnoise directory, they are:
99
100 - osnoise/cpus: CPUs at which a osnoise thread will execute.
101 - osnoise/period_us: the period of the osnoise thread.
102 - osnoise/runtime_us: how long an osnoise thread will look for noise.
103 - osnoise/stop_tracing_us: stop the system tracing if a single noise
104   higher than the configured value happens. Writing 0 disables this
105   option.
106 - osnoise/stop_tracing_total_us: stop the system tracing if total noise
107   higher than the configured value happens. Writing 0 disables this
108   option.
109 - tracing_threshold: the minimum delta between two time() reads to be
110   considered as noise, in us. When set to 0, the default value will
111   be used, which is currently 5 us.
112
113Additional Tracing
114------------------
115
116In addition to the tracer, a set of tracepoints were added to
117facilitate the identification of the osnoise source.
118
119 - osnoise:sample_threshold: printed anytime a noise is higher than
120   the configurable tolerance_ns.
121 - osnoise:nmi_noise: noise from NMI, including the duration.
122 - osnoise:irq_noise: noise from an IRQ, including the duration.
123 - osnoise:softirq_noise: noise from a SoftIRQ, including the
124   duration.
125 - osnoise:thread_noise: noise from a thread, including the duration.
126
127Note that all the values are *net values*. For example, if while osnoise
128is running, another thread preempts the osnoise thread, it will start a
129thread_noise duration at the start. Then, an IRQ takes place, preempting
130the thread_noise, starting a irq_noise. When the IRQ ends its execution,
131it will compute its duration, and this duration will be subtracted from
132the thread_noise, in such a way as to avoid the double accounting of the
133IRQ execution. This logic is valid for all sources of noise.
134
135Here is one example of the usage of these tracepoints::
136
137       osnoise/8-961     [008] d.h.  5789.857532: irq_noise: local_timer:236 start 5789.857529929 duration 1845 ns
138       osnoise/8-961     [008] dNh.  5789.858408: irq_noise: local_timer:236 start 5789.858404871 duration 2848 ns
139     migration/8-54      [008] d...  5789.858413: thread_noise: migration/8:54 start 5789.858409300 duration 3068 ns
140       osnoise/8-961     [008] ....  5789.858413: sample_threshold: start 5789.858404555 duration 8812 ns interferences 2
141
142In this example, a noise sample of 8 microseconds was reported in the last
143line, pointing to two interferences. Looking backward in the trace, the
144two previous entries were about the migration thread running after a
145timer IRQ execution. The first event is not part of the noise because
146it took place one millisecond before.
147
148It is worth noticing that the sum of the duration reported in the
149tracepoints is smaller than eight us reported in the sample_threshold.
150The reason roots in the overhead of the entry and exit code that happens
151before and after any interference execution. This justifies the dual
152approach: measuring thread and tracing.
153