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1 | RCU Torture Test Operation |
2 | ||
3 | ||
4 | CONFIG_RCU_TORTURE_TEST | |
5 | ||
6 | The CONFIG_RCU_TORTURE_TEST config option is available for all RCU | |
7 | implementations. It creates an rcutorture kernel module that can | |
8 | be loaded to run a torture test. The test periodically outputs | |
9 | status messages via printk(), which can be examined via the dmesg | |
72e9bb54 | 10 | command (perhaps grepping for "torture"). The test is started |
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11 | when the module is loaded, and stops when the module is unloaded. |
12 | ||
13 | However, actually setting this config option to "y" results in the system | |
14 | running the test immediately upon boot, and ending only when the system | |
15 | is taken down. Normally, one will instead want to build the system | |
16 | with CONFIG_RCU_TORTURE_TEST=m and to use modprobe and rmmod to control | |
17 | the test, perhaps using a script similar to the one shown at the end of | |
18 | this document. Note that you will need CONFIG_MODULE_UNLOAD in order | |
19 | to be able to end the test. | |
20 | ||
21 | ||
22 | MODULE PARAMETERS | |
23 | ||
24 | This module has the following parameters: | |
25 | ||
26 | nreaders This is the number of RCU reading threads supported. | |
27 | The default is twice the number of CPUs. Why twice? | |
28 | To properly exercise RCU implementations with preemptible | |
29 | read-side critical sections. | |
30 | ||
b772e1dd JT |
31 | nfakewriters This is the number of RCU fake writer threads to run. Fake |
32 | writer threads repeatedly use the synchronous "wait for | |
33 | current readers" function of the interface selected by | |
34 | torture_type, with a delay between calls to allow for various | |
35 | different numbers of writers running in parallel. | |
36 | nfakewriters defaults to 4, which provides enough parallelism | |
37 | to trigger special cases caused by multiple writers, such as | |
38 | the synchronize_srcu() early return optimization. | |
39 | ||
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40 | stat_interval The number of seconds between output of torture |
41 | statistics (via printk()). Regardless of the interval, | |
42 | statistics are printed when the module is unloaded. | |
43 | Setting the interval to zero causes the statistics to | |
44 | be printed -only- when the module is unloaded, and this | |
45 | is the default. | |
46 | ||
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47 | shuffle_interval |
48 | The number of seconds to keep the test threads affinitied | |
49 | to a particular subset of the CPUs. Used in conjunction | |
50 | with test_no_idle_hz. | |
51 | ||
52 | test_no_idle_hz Whether or not to test the ability of RCU to operate in | |
53 | a kernel that disables the scheduling-clock interrupt to | |
54 | idle CPUs. Boolean parameter, "1" to test, "0" otherwise. | |
55 | ||
20d2e428 JT |
56 | torture_type The type of RCU to test: "rcu" for the rcu_read_lock() API, |
57 | "rcu_sync" for rcu_read_lock() with synchronous reclamation, | |
11a14701 JT |
58 | "rcu_bh" for the rcu_read_lock_bh() API, "rcu_bh_sync" for |
59 | rcu_read_lock_bh() with synchronous reclamation, and "srcu" | |
60 | for the "srcu_read_lock()" API. | |
72e9bb54 | 61 | |
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62 | verbose Enable debug printk()s. Default is disabled. |
63 | ||
64 | ||
65 | OUTPUT | |
66 | ||
67 | The statistics output is as follows: | |
68 | ||
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69 | rcu-torture: --- Start of test: nreaders=16 stat_interval=0 verbose=0 |
70 | rcu-torture: rtc: 0000000000000000 ver: 1916 tfle: 0 rta: 1916 rtaf: 0 rtf: 1915 | |
71 | rcu-torture: Reader Pipe: 1466408 9747 0 0 0 0 0 0 0 0 0 | |
72 | rcu-torture: Reader Batch: 1464477 11678 0 0 0 0 0 0 0 0 | |
73 | rcu-torture: Free-Block Circulation: 1915 1915 1915 1915 1915 1915 1915 1915 1915 1915 0 | |
74 | rcu-torture: --- End of test | |
a241ec65 | 75 | |
72e9bb54 | 76 | The command "dmesg | grep torture:" will extract this information on |
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77 | most systems. On more esoteric configurations, it may be necessary to |
78 | use other commands to access the output of the printk()s used by | |
79 | the RCU torture test. The printk()s use KERN_ALERT, so they should | |
80 | be evident. ;-) | |
81 | ||
82 | The entries are as follows: | |
83 | ||
84 | o "ggp": The number of counter flips (or batches) since boot. | |
85 | ||
86 | o "rtc": The hexadecimal address of the structure currently visible | |
87 | to readers. | |
88 | ||
89 | o "ver": The number of times since boot that the rcutw writer task | |
90 | has changed the structure visible to readers. | |
91 | ||
92 | o "tfle": If non-zero, indicates that the "torture freelist" | |
93 | containing structure to be placed into the "rtc" area is empty. | |
94 | This condition is important, since it can fool you into thinking | |
95 | that RCU is working when it is not. :-/ | |
96 | ||
97 | o "rta": Number of structures allocated from the torture freelist. | |
98 | ||
99 | o "rtaf": Number of allocations from the torture freelist that have | |
100 | failed due to the list being empty. | |
101 | ||
102 | o "rtf": Number of frees into the torture freelist. | |
103 | ||
104 | o "Reader Pipe": Histogram of "ages" of structures seen by readers. | |
105 | If any entries past the first two are non-zero, RCU is broken. | |
106 | And rcutorture prints the error flag string "!!!" to make sure | |
107 | you notice. The age of a newly allocated structure is zero, | |
108 | it becomes one when removed from reader visibility, and is | |
109 | incremented once per grace period subsequently -- and is freed | |
110 | after passing through (RCU_TORTURE_PIPE_LEN-2) grace periods. | |
111 | ||
112 | The output displayed above was taken from a correctly working | |
113 | RCU. If you want to see what it looks like when broken, break | |
114 | it yourself. ;-) | |
115 | ||
116 | o "Reader Batch": Another histogram of "ages" of structures seen | |
117 | by readers, but in terms of counter flips (or batches) rather | |
118 | than in terms of grace periods. The legal number of non-zero | |
119 | entries is again two. The reason for this separate view is | |
120 | that it is easier to get the third entry to show up in the | |
121 | "Reader Batch" list than in the "Reader Pipe" list. | |
122 | ||
123 | o "Free-Block Circulation": Shows the number of torture structures | |
124 | that have reached a given point in the pipeline. The first element | |
125 | should closely correspond to the number of structures allocated, | |
126 | the second to the number that have been removed from reader view, | |
127 | and all but the last remaining to the corresponding number of | |
128 | passes through a grace period. The last entry should be zero, | |
129 | as it is only incremented if a torture structure's counter | |
130 | somehow gets incremented farther than it should. | |
131 | ||
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132 | Different implementations of RCU can provide implementation-specific |
133 | additional information. For example, SRCU provides the following: | |
134 | ||
135 | srcu-torture: rtc: f8cf46a8 ver: 355 tfle: 0 rta: 356 rtaf: 0 rtf: 346 rtmbe: 0 | |
136 | srcu-torture: Reader Pipe: 559738 939 0 0 0 0 0 0 0 0 0 | |
137 | srcu-torture: Reader Batch: 560434 243 0 0 0 0 0 0 0 0 | |
138 | srcu-torture: Free-Block Circulation: 355 354 353 352 351 350 349 348 347 346 0 | |
139 | srcu-torture: per-CPU(idx=1): 0(0,1) 1(0,1) 2(0,0) 3(0,1) | |
140 | ||
141 | The first four lines are similar to those for RCU. The last line shows | |
142 | the per-CPU counter state. The numbers in parentheses are the values | |
143 | of the "old" and "current" counters for the corresponding CPU. The | |
144 | "idx" value maps the "old" and "current" values to the underlying array, | |
145 | and is useful for debugging. | |
146 | ||
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147 | |
148 | USAGE | |
149 | ||
150 | The following script may be used to torture RCU: | |
151 | ||
152 | #!/bin/sh | |
153 | ||
154 | modprobe rcutorture | |
155 | sleep 100 | |
156 | rmmod rcutorture | |
72e9bb54 | 157 | dmesg | grep torture: |
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158 | |
159 | The output can be manually inspected for the error flag of "!!!". | |
160 | One could of course create a more elaborate script that automatically | |
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161 | checked for such errors. The "rmmod" command forces a "SUCCESS" or |
162 | "FAILURE" indication to be printk()ed. |