1 ========================= 2 Hardware Latency Detector 3 ========================= 4 5 Introduction 6 ------------- 7 8 The tracer hwlat_detector is a special purpose tracer that is used to 9 detect large system latencies induced by the behavior of certain underlying 10 hardware or firmware, independent of Linux itself. The code was developed 11 originally to detect SMIs (System Management Interrupts) on x86 systems, 12 however there is nothing x86 specific about this patchset. It was 13 originally written for use by the "RT" patch since the Real Time 14 kernel is highly latency sensitive. 15 16 SMIs are not serviced by the Linux kernel, which means that it does not 17 even know that they are occurring. SMIs are instead set up by BIOS code 18 and are serviced by BIOS code, usually for "critical" events such as 19 management of thermal sensors and fans. Sometimes though, SMIs are used for 20 other tasks and those tasks can spend an inordinate amount of time in the 21 handler (sometimes measured in milliseconds). Obviously this is a problem if 22 you are trying to keep event service latencies down in the microsecond range. 23 24 The hardware latency detector works by hogging one of the cpus for configurable 25 amounts of time (with interrupts disabled), polling the CPU Time Stamp Counter 26 for some period, then looking for gaps in the TSC data. Any gap indicates a 27 time when the polling was interrupted and since the interrupts are disabled, 28 the only thing that could do that would be an SMI or other hardware hiccup 29 (or an NMI, but those can be tracked). 30 31 Note that the hwlat detector should *NEVER* be used in a production environment. 32 It is intended to be run manually to determine if the hardware platform has a 33 problem with long system firmware service routines. 34 35 Usage 36 ------ 37 38 Write the ASCII text "hwlat" into the current_tracer file of the tracing system 39 (mounted at /sys/kernel/tracing or /sys/kernel/tracing). It is possible to 40 redefine the threshold in microseconds (us) above which latency spikes will 41 be taken into account. 42 43 Example:: 44 45 # echo hwlat > /sys/kernel/tracing/current_tracer 46 # echo 100 > /sys/kernel/tracing/tracing_thresh 47 48 The /sys/kernel/tracing/hwlat_detector interface contains the following files: 49 50 - width - time period to sample with CPUs held (usecs) 51 must be less than the total window size (enforced) 52 - window - total period of sampling, width being inside (usecs) 53 54 By default the width is set to 500,000 and window to 1,000,000, meaning that 55 for every 1,000,000 usecs (1s) the hwlat detector will spin for 500,000 usecs 56 (0.5s). If tracing_thresh contains zero when hwlat tracer is enabled, it will 57 change to a default of 10 usecs. If any latencies that exceed the threshold is 58 observed then the data will be written to the tracing ring buffer. 59 60 The minimum sleep time between periods is 1 millisecond. Even if width 61 is less than 1 millisecond apart from window, to allow the system to not 62 be totally starved. 63 64 If tracing_thresh was zero when hwlat detector was started, it will be set 65 back to zero if another tracer is loaded. Note, the last value in 66 tracing_thresh that hwlat detector had will be saved and this value will 67 be restored in tracing_thresh if it is still zero when hwlat detector is 68 started again. 69 70 The following tracing directory files are used by the hwlat_detector: 71 72 in /sys/kernel/tracing: 73 74 - tracing_threshold - minimum latency value to be considered (usecs) 75 - tracing_max_latency - maximum hardware latency actually observed (usecs) 76 - tracing_cpumask - the CPUs to move the hwlat thread across 77 - hwlat_detector/width - specified amount of time to spin within window (usecs) 78 - hwlat_detector/window - amount of time between (width) runs (usecs) 79 - hwlat_detector/mode - the thread mode 80 81 By default, one hwlat detector's kernel thread will migrate across each CPU 82 specified in cpumask at the beginning of a new window, in a round-robin 83 fashion. This behavior can be changed by changing the thread mode, 84 the available options are: 85 86 - none: do not force migration 87 - round-robin: migrate across each CPU specified in cpumask [default] 88 - per-cpu: create one thread for each cpu in tracing_cpumask
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