1 .. SPDX-License-Identifier: GPL-2.0
2
3 ==================================
4 Tracefs ring-buffer memory mapping
5 ==================================
6
7 :Author: Vincent Donnefort <vdonnefort@google.c
8
9 Overview
10 ========
11 Tracefs ring-buffer memory map provides an eff
12 as no memory copy is necessary. The applicatio
13 then a consumer for that ring-buffer, in a sim
14
15 Memory mapping setup
16 ====================
17 The mapping works with a mmap() of the trace_p
18
19 The first system page of the mapping contains
20 description. It is referred to as the meta-pag
21 fields of the meta-page is the reader. It cont
22 be safely read by the mapper (see ring-buffer-
23
24 The meta-page is followed by all the sub-buffe
25 therefore effortless to know where the reader
26
27 .. code-block:: c
28
29 reader_id = meta->reader->id;
30 reader_offset = meta->meta_page_size +
31
32 When the application is done with the current
33 the trace_pipe_raw ioctl() TRACE_MMAP_IOCTL_GE
34 the meta-page fields.
35
36 Limitations
37 ===========
38 When a mapping is in place on a Tracefs ring-b
39 either resize it (either by increasing the ent
40 each subbuf). It is also not possible to use s
41 the ring buffer data instead of using the copy
42
43 Concurrent readers (either another application
44 kernel with trace_pipe) are allowed but not re
45 the ring-buffer and the output is unpredictabl
46 trace_pipe would be.
47
48 Example
49 =======
50
51 .. code-block:: c
52
53 #include <fcntl.h>
54 #include <stdio.h>
55 #include <stdlib.h>
56 #include <unistd.h>
57
58 #include <linux/trace_mmap.h>
59
60 #include <sys/mman.h>
61 #include <sys/ioctl.h>
62
63 #define TRACE_PIPE_RAW "/sys/kernel/tr
64
65 int main(void)
66 {
67 int page_size = getpagesize(),
68 unsigned long meta_len, data_l
69 struct trace_buffer_meta *meta
70 void *map, *reader, *data;
71
72 fd = open(TRACE_PIPE_RAW, O_RD
73 if (fd < 0)
74 exit(EXIT_FAILURE);
75
76 map = mmap(NULL, page_size, PR
77 if (map == MAP_FAILED)
78 exit(EXIT_FAILURE);
79
80 meta = (struct trace_buffer_me
81 meta_len = meta->meta_page_siz
82
83 printf("entries: %llu\n
84 printf("overrun: %llu\n
85 printf("read: %llu\n
86 printf("nr_subbufs: %u\n",
87
88 data_len = meta->subbuf_size *
89 data = mmap(NULL, data_len, PR
90 if (data == MAP_FAILED)
91 exit(EXIT_FAILURE);
92
93 if (ioctl(fd, TRACE_MMAP_IOCTL
94 exit(EXIT_FAILURE);
95
96 reader_id = meta->reader.id;
97 reader = data + meta->subbuf_s
98
99 printf("Current reader address
100
101 munmap(data, data_len);
102 munmap(meta, meta_len);
103 close (fd);
104
105 return 0;
106 }
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