1HWLATDETECT(8) System Manager's Manual HWLATDETECT(8)
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6 hwlatdetect - program to control the ftrace kernel hardware latency de‐
7 tection
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10 hwlatdetect [ --duration=<time> ] [--threshold=<usecs> ] [--win‐
11 dow=<time interval> ] [--width=<time interval> ] [--hardlimit=<mi‐
12 crosecond value> ] [--report=<path> ] [--debug ] [--quiet ] [--watch]
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16 hwlatdetect is a program that controls the ftrace kernel hardware la‐
17 tency detector (hwlatdetector).hwlatdetector is used to detect large
18 system latencies induced by the behavior of certain underlying hardware
19 or firmware, independent of Linux itself. The code was developed orig‐
20 inally to detect SMIs (System Management Interrupts) on x86 systems,
21 however there is nothing x86 specific about it. It was originally
22 written for use by the "RT" patch set since the Real Time kernel is
23 highly latency sensitive.
24 SMIs are usually not serviced by the Linux kernel, which typically does
26 not even know that they are occurring. SMIs are instead are set up by
27 BIOS code and are serviced by BIOS code, usually for "critical" events
28 such as management of thermal sensors and fans. Sometimes though, SMIs
29 are used for other tasks and those tasks can spend an inordinate amount
30 of time in the handler (sometimes measured in milliseconds). Obviously
31 this is a problem if you are trying to keep event service latencies
32 down in the microsecond range.
33 The ftrace hardware latency detector works by hogging all of the cpus
35 for configurable amounts of time (by calling stop_machine()), polling
36 the CPU Time Stamp Counter for some period, then looking for gaps in
37 the TSC data. Any gap indicates a time when the polling was interrupted
38 and since the machine is stopped and interrupts turned off the only
39 thing that could do that would be an SMI.
40 The hwlatdetector script manages the mounting/unmounting of the debugfs
42 as well as interacting with the ftrace hwlatdetector If the debugfs is
43 already mounted then hwlatdetector will not unmount it after a run.
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47 --duration=<time>{s,m,d}
48 Run the detector logic in for the specified duration. The dura‐
49 tion is a base 10 integer number that defaults to a value in
50 seconds. An optional suffix may be specified to indicate min‐
51 utes, hours or days.
52 --threshold=<microsecond value>
55 Specify the TSC gap used to detect an SMI. Any gap value greater
56 than <threshold> is considered to be the result of an SMI occur‐
57 ring.
58 --hardlimit=<microsecond value>
61 The test is considered to fail if a value above the hardlimit
62 occurs. This affects the exit value of hwlatdetect
63 --window=<time value>{us,ms,s,m,d}
66 specify the size of the sample window. Converted to microseconds
67 when passed to the kernel.
68 --width=<time value>{us,ms,s,m,d}
71 The amount of time within the sample window where the detector
72 is actually sampling. Must be less than the --window value.
73 --report=FILENAME
75 Specify the output filename of the detector report. Default be‐
76 havior is to print to standard output
77 --cpu-list=CPU-LIST
79 Specify the CPUs for hwlat thread to move across.
80 --debug
82 Turn on debug prints
83 --quiet
85 Turn off all information prints
86 --watch
88 print sample data to stdout as it arrives
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92 hwlatdetect was written by Clark Williams <williams@redhat.com>
93 hwlat_detector.ko was written by Jon Masters <jcm@redhat.com>
94 May 12, 2009 HWLATDETECT(8)