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Documentation / trace / hwlat_detector.txt




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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	Introduction:
2	-------------
3	
4	The tracer hwlat_detector is a special purpose tracer that is used to
5	detect large system latencies induced by the behavior of certain underlying
6	hardware or firmware, independent of Linux itself. The code was developed
7	originally to detect SMIs (System Management Interrupts) on x86 systems,
8	however there is nothing x86 specific about this patchset. It was
9	originally written for use by the "RT" patch since the Real Time
10	kernel is highly latency sensitive.
11	
12	SMIs are not serviced by the Linux kernel, which means that it does not
13	even know that they are occuring. SMIs are instead set up by BIOS code
14	and are serviced by BIOS code, usually for "critical" events such as
15	management of thermal sensors and fans. Sometimes though, SMIs are used for
16	other tasks and those tasks can spend an inordinate amount of time in the
17	handler (sometimes measured in milliseconds). Obviously this is a problem if
18	you are trying to keep event service latencies down in the microsecond range.
19	
20	The hardware latency detector works by hogging one of the cpus for configurable
21	amounts of time (with interrupts disabled), polling the CPU Time Stamp Counter
22	for some period, then looking for gaps in the TSC data. Any gap indicates a
23	time when the polling was interrupted and since the interrupts are disabled,
24	the only thing that could do that would be an SMI or other hardware hiccup
25	(or an NMI, but those can be tracked).
26	
27	Note that the hwlat detector should *NEVER* be used in a production environment.
28	It is intended to be run manually to determine if the hardware platform has a
29	problem with long system firmware service routines.
30	
31	Usage:
32	------
33	
34	Write the ASCII text "hwlat" into the current_tracer file of the tracing system
35	(mounted at /sys/kernel/tracing or /sys/kernel/tracing). It is possible to
36	redefine the threshold in microseconds (us) above which latency spikes will
37	be taken into account.
38	
39	Example:
40	
41		# echo hwlat > /sys/kernel/tracing/current_tracer
42		# echo 100 > /sys/kernel/tracing/tracing_thresh
43	
44	The /sys/kernel/tracing/hwlat_detector interface contains the following files:
45	
46	width			- time period to sample with CPUs held (usecs)
47				  must be less than the total window size (enforced)
48	window			- total period of sampling, width being inside (usecs)
49	
50	By default the width is set to 500,000 and window to 1,000,000, meaning that
51	for every 1,000,000 usecs (1s) the hwlat detector will spin for 500,000 usecs
52	(0.5s). If tracing_thresh contains zero when hwlat tracer is enabled, it will
53	change to a default of 10 usecs. If any latencies that exceed the threshold is
54	observed then the data will be written to the tracing ring buffer.
55	
56	The minimum sleep time between periods is 1 millisecond. Even if width
57	is less than 1 millisecond apart from window, to allow the system to not
58	be totally starved.
59	
60	If tracing_thresh was zero when hwlat detector was started, it will be set
61	back to zero if another tracer is loaded. Note, the last value in
62	tracing_thresh that hwlat detector had will be saved and this value will
63	be restored in tracing_thresh if it is still zero when hwlat detector is
64	started again.
65	
66	The following tracing directory files are used by the hwlat_detector:
67	
68	in /sys/kernel/tracing:
69	
70	 tracing_threshold	- minimum latency value to be considered (usecs)
71	 tracing_max_latency	- maximum hardware latency actually observed (usecs)
72	 tracing_cpumask	- the CPUs to move the hwlat thread across
73	 hwlat_detector/width	- specified amount of time to spin within window (usecs)
74	 hwlat_detector/window	- amount of time between (width) runs (usecs)
75	
76	The hwlat detector's kernel thread will migrate across each CPU specified in
77	tracing_cpumask between each window. To limit the migration, either modify
78	tracing_cpumask, or modify the hwlat kernel thread (named [hwlatd]) CPU
79	affinity directly, and the migration will stop.
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