1 ================ 1 ================
2 Delay accounting 2 Delay accounting
3 ================ 3 ================
4 4
5 Tasks encounter delays in execution when they 5 Tasks encounter delays in execution when they wait
6 for some kernel resource to become available e 6 for some kernel resource to become available e.g. a
7 runnable task may wait for a free CPU to run o 7 runnable task may wait for a free CPU to run on.
8 8
9 The per-task delay accounting functionality me 9 The per-task delay accounting functionality measures
10 the delays experienced by a task while 10 the delays experienced by a task while
11 11
12 a) waiting for a CPU (while being runnable) 12 a) waiting for a CPU (while being runnable)
13 b) completion of synchronous block I/O initiat 13 b) completion of synchronous block I/O initiated by the task
14 c) swapping in pages 14 c) swapping in pages
15 d) memory reclaim 15 d) memory reclaim
16 e) thrashing !! 16 e) thrashing page cache
17 f) direct compact 17 f) direct compact
18 g) write-protect copy <<
19 h) IRQ/SOFTIRQ <<
20 18
21 and makes these statistics available to usersp 19 and makes these statistics available to userspace through
22 the taskstats interface. 20 the taskstats interface.
23 21
24 Such delays provide feedback for setting a tas 22 Such delays provide feedback for setting a task's cpu priority,
25 io priority and rss limit values appropriately 23 io priority and rss limit values appropriately. Long delays for
26 important tasks could be a trigger for raising 24 important tasks could be a trigger for raising its corresponding priority.
27 25
28 The functionality, through its use of the task 26 The functionality, through its use of the taskstats interface, also provides
29 delay statistics aggregated for all tasks (or 27 delay statistics aggregated for all tasks (or threads) belonging to a
30 thread group (corresponding to a traditional U 28 thread group (corresponding to a traditional Unix process). This is a commonly
31 needed aggregation that is more efficiently do 29 needed aggregation that is more efficiently done by the kernel.
32 30
33 Userspace utilities, particularly resource man 31 Userspace utilities, particularly resource management applications, can also
34 aggregate delay statistics into arbitrary grou 32 aggregate delay statistics into arbitrary groups. To enable this, delay
35 statistics of a task are available both during 33 statistics of a task are available both during its lifetime as well as on its
36 exit, ensuring continuous and complete monitor 34 exit, ensuring continuous and complete monitoring can be done.
37 35
38 36
39 Interface 37 Interface
40 --------- 38 ---------
41 39
42 Delay accounting uses the taskstats interface 40 Delay accounting uses the taskstats interface which is described
43 in detail in a separate document in this direc 41 in detail in a separate document in this directory. Taskstats returns a
44 generic data structure to userspace correspond 42 generic data structure to userspace corresponding to per-pid and per-tgid
45 statistics. The delay accounting functionality 43 statistics. The delay accounting functionality populates specific fields of
46 this structure. See 44 this structure. See
47 45
48 include/uapi/linux/taskstats.h 46 include/uapi/linux/taskstats.h
49 47
50 for a description of the fields pertaining to 48 for a description of the fields pertaining to delay accounting.
51 It will generally be in the form of counters r 49 It will generally be in the form of counters returning the cumulative
52 delay seen for cpu, sync block I/O, swapin, me 50 delay seen for cpu, sync block I/O, swapin, memory reclaim, thrash page
53 cache, direct compact, write-protect copy, IRQ !! 51 cache, direct compact etc.
54 52
55 Taking the difference of two successive readin 53 Taking the difference of two successive readings of a given
56 counter (say cpu_delay_total) for a task will 54 counter (say cpu_delay_total) for a task will give the delay
57 experienced by the task waiting for the corres 55 experienced by the task waiting for the corresponding resource
58 in that interval. 56 in that interval.
59 57
60 When a task exits, records containing the per- 58 When a task exits, records containing the per-task statistics
61 are sent to userspace without requiring a comm 59 are sent to userspace without requiring a command. If it is the last exiting
62 task of a thread group, the per-tgid statistic 60 task of a thread group, the per-tgid statistics are also sent. More details
63 are given in the taskstats interface descripti 61 are given in the taskstats interface description.
64 62
65 The getdelays.c userspace utility in tools/acc 63 The getdelays.c userspace utility in tools/accounting directory allows simple
66 commands to be run and the corresponding delay 64 commands to be run and the corresponding delay statistics to be displayed. It
67 also serves as an example of using the tasksta 65 also serves as an example of using the taskstats interface.
68 66
69 Usage 67 Usage
70 ----- 68 -----
71 69
72 Compile the kernel with:: 70 Compile the kernel with::
73 71
74 CONFIG_TASK_DELAY_ACCT=y 72 CONFIG_TASK_DELAY_ACCT=y
75 CONFIG_TASKSTATS=y 73 CONFIG_TASKSTATS=y
76 74
77 Delay accounting is disabled by default at boo 75 Delay accounting is disabled by default at boot up.
78 To enable, add:: 76 To enable, add::
79 77
80 delayacct 78 delayacct
81 79
82 to the kernel boot options. The rest of the in 80 to the kernel boot options. The rest of the instructions below assume this has
83 been done. Alternatively, use sysctl kernel.ta 81 been done. Alternatively, use sysctl kernel.task_delayacct to switch the state
84 at runtime. Note however that only tasks start 82 at runtime. Note however that only tasks started after enabling it will have
85 delayacct information. 83 delayacct information.
86 84
87 After the system has booted up, use a utility 85 After the system has booted up, use a utility
88 similar to getdelays.c to access the delays 86 similar to getdelays.c to access the delays
89 seen by a given task or a task group (tgid). 87 seen by a given task or a task group (tgid).
90 The utility also allows a given command to be 88 The utility also allows a given command to be
91 executed and the corresponding delays to be 89 executed and the corresponding delays to be
92 seen. 90 seen.
93 91
94 General format of the getdelays command:: 92 General format of the getdelays command::
95 93
96 getdelays [-dilv] [-t tgid] [-p pid] 94 getdelays [-dilv] [-t tgid] [-p pid]
97 95
98 Get delays, since system boot, for pid 10:: 96 Get delays, since system boot, for pid 10::
99 97
100 # ./getdelays -d -p 10 98 # ./getdelays -d -p 10
101 (output similar to next case) 99 (output similar to next case)
102 100
103 Get sum of delays, since system boot, for all 101 Get sum of delays, since system boot, for all pids with tgid 5::
104 102
105 # ./getdelays -d -t 5 103 # ./getdelays -d -t 5
106 print delayacct stats ON 104 print delayacct stats ON
107 TGID 5 105 TGID 5
108 106
109 107
110 CPU count real total 108 CPU count real total virtual total delay total delay average
111 8 7000000 109 8 7000000 6872122 3382277 0.423ms
112 IO count delay total 110 IO count delay total delay average
113 0 0 0 !! 111 0 0 0ms
114 SWAP count delay total 112 SWAP count delay total delay average
115 0 0 !! 113 0 0 0ms
116 RECLAIM count delay total 114 RECLAIM count delay total delay average
117 0 0 0 !! 115 0 0 0ms
118 THRASHING count delay total 116 THRASHING count delay total delay average
119 0 0 !! 117 0 0 0ms
120 COMPACT count delay total 118 COMPACT count delay total delay average
121 0 0 !! 119 0 0 0ms
122 WPCOPY count delay total <<
123 0 0 <<
124 IRQ count delay total <<
125 0 0 <<
126 120
127 Get IO accounting for pid 1, it works only wit 121 Get IO accounting for pid 1, it works only with -p::
128 122
129 # ./getdelays -i -p 1 123 # ./getdelays -i -p 1
130 printing IO accounting 124 printing IO accounting
131 linuxrc: read=65536, write=0, cancelle 125 linuxrc: read=65536, write=0, cancelled_write=0
132 126
133 The above command can be used with -v to get m 127 The above command can be used with -v to get more debug information.
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