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f7be8610 BP |
1 | x86 Topology |
2 | ============ | |
3 | ||
4 | This documents and clarifies the main aspects of x86 topology modelling and | |
5 | representation in the kernel. Update/change when doing changes to the | |
6 | respective code. | |
7 | ||
8 | The architecture-agnostic topology definitions are in | |
9 | Documentation/cputopology.txt. This file holds x86-specific | |
10 | differences/specialities which must not necessarily apply to the generic | |
11 | definitions. Thus, the way to read up on Linux topology on x86 is to start | |
12 | with the generic one and look at this one in parallel for the x86 specifics. | |
13 | ||
14 | Needless to say, code should use the generic functions - this file is *only* | |
15 | here to *document* the inner workings of x86 topology. | |
16 | ||
17 | Started by Thomas Gleixner <tglx@linutronix.de> and Borislav Petkov <bp@alien8.de>. | |
18 | ||
19 | The main aim of the topology facilities is to present adequate interfaces to | |
20 | code which needs to know/query/use the structure of the running system wrt | |
21 | threads, cores, packages, etc. | |
22 | ||
23 | The kernel does not care about the concept of physical sockets because a | |
24 | socket has no relevance to software. It's an electromechanical component. In | |
25 | the past a socket always contained a single package (see below), but with the | |
26 | advent of Multi Chip Modules (MCM) a socket can hold more than one package. So | |
27 | there might be still references to sockets in the code, but they are of | |
28 | historical nature and should be cleaned up. | |
29 | ||
30 | The topology of a system is described in the units of: | |
31 | ||
32 | - packages | |
33 | - cores | |
34 | - threads | |
35 | ||
36 | * Package: | |
37 | ||
38 | Packages contain a number of cores plus shared resources, e.g. DRAM | |
39 | controller, shared caches etc. | |
40 | ||
41 | AMD nomenclature for package is 'Node'. | |
42 | ||
43 | Package-related topology information in the kernel: | |
44 | ||
45 | - cpuinfo_x86.x86_max_cores: | |
46 | ||
47 | The number of cores in a package. This information is retrieved via CPUID. | |
48 | ||
49 | - cpuinfo_x86.phys_proc_id: | |
50 | ||
51 | The physical ID of the package. This information is retrieved via CPUID | |
52 | and deduced from the APIC IDs of the cores in the package. | |
53 | ||
54 | - cpuinfo_x86.logical_id: | |
55 | ||
56 | The logical ID of the package. As we do not trust BIOSes to enumerate the | |
57 | packages in a consistent way, we introduced the concept of logical package | |
58 | ID so we can sanely calculate the number of maximum possible packages in | |
59 | the system and have the packages enumerated linearly. | |
60 | ||
61 | - topology_max_packages(): | |
62 | ||
63 | The maximum possible number of packages in the system. Helpful for per | |
64 | package facilities to preallocate per package information. | |
65 | ||
66 | ||
67 | * Cores: | |
68 | ||
69 | A core consists of 1 or more threads. It does not matter whether the threads | |
70 | are SMT- or CMT-type threads. | |
71 | ||
72 | AMDs nomenclature for a CMT core is "Compute Unit". The kernel always uses | |
73 | "core". | |
74 | ||
75 | Core-related topology information in the kernel: | |
76 | ||
77 | - smp_num_siblings: | |
78 | ||
79 | The number of threads in a core. The number of threads in a package can be | |
80 | calculated by: | |
81 | ||
82 | threads_per_package = cpuinfo_x86.x86_max_cores * smp_num_siblings | |
83 | ||
84 | ||
85 | * Threads: | |
86 | ||
87 | A thread is a single scheduling unit. It's the equivalent to a logical Linux | |
88 | CPU. | |
89 | ||
90 | AMDs nomenclature for CMT threads is "Compute Unit Core". The kernel always | |
91 | uses "thread". | |
92 | ||
93 | Thread-related topology information in the kernel: | |
94 | ||
95 | - topology_core_cpumask(): | |
96 | ||
97 | The cpumask contains all online threads in the package to which a thread | |
98 | belongs. | |
99 | ||
100 | The number of online threads is also printed in /proc/cpuinfo "siblings." | |
101 | ||
102 | - topology_sibling_mask(): | |
103 | ||
104 | The cpumask contains all online threads in the core to which a thread | |
105 | belongs. | |
106 | ||
107 | - topology_logical_package_id(): | |
108 | ||
109 | The logical package ID to which a thread belongs. | |
110 | ||
111 | - topology_physical_package_id(): | |
112 | ||
113 | The physical package ID to which a thread belongs. | |
114 | ||
115 | - topology_core_id(); | |
116 | ||
117 | The ID of the core to which a thread belongs. It is also printed in /proc/cpuinfo | |
118 | "core_id." | |
119 | ||
120 | ||
121 | ||
122 | System topology examples | |
123 | ||
124 | Note: | |
125 | ||
126 | The alternative Linux CPU enumeration depends on how the BIOS enumerates the | |
127 | threads. Many BIOSes enumerate all threads 0 first and then all threads 1. | |
128 | That has the "advantage" that the logical Linux CPU numbers of threads 0 stay | |
129 | the same whether threads are enabled or not. That's merely an implementation | |
130 | detail and has no practical impact. | |
131 | ||
132 | 1) Single Package, Single Core | |
133 | ||
134 | [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 | |
135 | ||
136 | 2) Single Package, Dual Core | |
137 | ||
138 | a) One thread per core | |
139 | ||
140 | [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 | |
141 | -> [core 1] -> [thread 0] -> Linux CPU 1 | |
142 | ||
143 | b) Two threads per core | |
144 | ||
145 | [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 | |
146 | -> [thread 1] -> Linux CPU 1 | |
147 | -> [core 1] -> [thread 0] -> Linux CPU 2 | |
148 | -> [thread 1] -> Linux CPU 3 | |
149 | ||
150 | Alternative enumeration: | |
151 | ||
152 | [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 | |
153 | -> [thread 1] -> Linux CPU 2 | |
154 | -> [core 1] -> [thread 0] -> Linux CPU 1 | |
155 | -> [thread 1] -> Linux CPU 3 | |
156 | ||
157 | AMD nomenclature for CMT systems: | |
158 | ||
159 | [node 0] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 0 | |
160 | -> [Compute Unit Core 1] -> Linux CPU 1 | |
161 | -> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 2 | |
162 | -> [Compute Unit Core 1] -> Linux CPU 3 | |
163 | ||
164 | 4) Dual Package, Dual Core | |
165 | ||
166 | a) One thread per core | |
167 | ||
168 | [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 | |
169 | -> [core 1] -> [thread 0] -> Linux CPU 1 | |
170 | ||
171 | [package 1] -> [core 0] -> [thread 0] -> Linux CPU 2 | |
172 | -> [core 1] -> [thread 0] -> Linux CPU 3 | |
173 | ||
174 | b) Two threads per core | |
175 | ||
176 | [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 | |
177 | -> [thread 1] -> Linux CPU 1 | |
178 | -> [core 1] -> [thread 0] -> Linux CPU 2 | |
179 | -> [thread 1] -> Linux CPU 3 | |
180 | ||
181 | [package 1] -> [core 0] -> [thread 0] -> Linux CPU 4 | |
182 | -> [thread 1] -> Linux CPU 5 | |
183 | -> [core 1] -> [thread 0] -> Linux CPU 6 | |
184 | -> [thread 1] -> Linux CPU 7 | |
185 | ||
186 | Alternative enumeration: | |
187 | ||
188 | [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 | |
189 | -> [thread 1] -> Linux CPU 4 | |
190 | -> [core 1] -> [thread 0] -> Linux CPU 1 | |
191 | -> [thread 1] -> Linux CPU 5 | |
192 | ||
193 | [package 1] -> [core 0] -> [thread 0] -> Linux CPU 2 | |
194 | -> [thread 1] -> Linux CPU 6 | |
195 | -> [core 1] -> [thread 0] -> Linux CPU 3 | |
196 | -> [thread 1] -> Linux CPU 7 | |
197 | ||
198 | AMD nomenclature for CMT systems: | |
199 | ||
200 | [node 0] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 0 | |
201 | -> [Compute Unit Core 1] -> Linux CPU 1 | |
202 | -> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 2 | |
203 | -> [Compute Unit Core 1] -> Linux CPU 3 | |
204 | ||
205 | [node 1] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 4 | |
206 | -> [Compute Unit Core 1] -> Linux CPU 5 | |
207 | -> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 6 | |
208 | -> [Compute Unit Core 1] -> Linux CPU 7 |