1.. SPDX-License-Identifier: GPL-2.0
2
3============
4x86 Topology
5============
6
7This documents and clarifies the main aspects of x86 topology modelling and
8representation in the kernel. Update/change when doing changes to the
9respective code.
10
11The architecture-agnostic topology definitions are in
12Documentation/admin-guide/cputopology.rst. This file holds x86-specific
13differences/specialities which must not necessarily apply to the generic
14definitions. Thus, the way to read up on Linux topology on x86 is to start
15with the generic one and look at this one in parallel for the x86 specifics.
16
17Needless to say, code should use the generic functions - this file is *only*
18here to *document* the inner workings of x86 topology.
19
20Started by Thomas Gleixner <tglx@linutronix.de> and Borislav Petkov <bp@alien8.de>.
21
22The main aim of the topology facilities is to present adequate interfaces to
23code which needs to know/query/use the structure of the running system wrt
24threads, cores, packages, etc.
25
26The kernel does not care about the concept of physical sockets because a
27socket has no relevance to software. It's an electromechanical component. In
28the past a socket always contained a single package (see below), but with the
29advent of Multi Chip Modules (MCM) a socket can hold more than one package. So
30there might be still references to sockets in the code, but they are of
31historical nature and should be cleaned up.
32
33The topology of a system is described in the units of:
34
35    - packages
36    - cores
37    - threads
38
39Package
40=======
41Packages contain a number of cores plus shared resources, e.g. DRAM
42controller, shared caches etc.
43
44AMD nomenclature for package is 'Node'.
45
46Package-related topology information in the kernel:
47
48  - cpuinfo_x86.x86_max_cores:
49
50    The number of cores in a package. This information is retrieved via CPUID.
51
52  - cpuinfo_x86.x86_max_dies:
53
54    The number of dies in a package. This information is retrieved via CPUID.
55
56  - cpuinfo_x86.phys_proc_id:
57
58    The physical ID of the package. This information is retrieved via CPUID
59    and deduced from the APIC IDs of the cores in the package.
60
61  - cpuinfo_x86.logical_proc_id:
62
63    The logical ID of the package. As we do not trust BIOSes to enumerate the
64    packages in a consistent way, we introduced the concept of logical package
65    ID so we can sanely calculate the number of maximum possible packages in
66    the system and have the packages enumerated linearly.
67
68  - topology_max_packages():
69
70    The maximum possible number of packages in the system. Helpful for per
71    package facilities to preallocate per package information.
72
73  - cpu_llc_id:
74
75    A per-CPU variable containing:
76
77      - On Intel, the first APIC ID of the list of CPUs sharing the Last Level
78        Cache
79
80      - On AMD, the Node ID or Core Complex ID containing the Last Level
81        Cache. In general, it is a number identifying an LLC uniquely on the
82        system.
83
84Cores
85=====
86A core consists of 1 or more threads. It does not matter whether the threads
87are SMT- or CMT-type threads.
88
89AMDs nomenclature for a CMT core is "Compute Unit". The kernel always uses
90"core".
91
92Core-related topology information in the kernel:
93
94  - smp_num_siblings:
95
96    The number of threads in a core. The number of threads in a package can be
97    calculated by::
98
99	threads_per_package = cpuinfo_x86.x86_max_cores * smp_num_siblings
100
101
102Threads
103=======
104A thread is a single scheduling unit. It's the equivalent to a logical Linux
105CPU.
106
107AMDs nomenclature for CMT threads is "Compute Unit Core". The kernel always
108uses "thread".
109
110Thread-related topology information in the kernel:
111
112  - topology_core_cpumask():
113
114    The cpumask contains all online threads in the package to which a thread
115    belongs.
116
117    The number of online threads is also printed in /proc/cpuinfo "siblings."
118
119  - topology_sibling_cpumask():
120
121    The cpumask contains all online threads in the core to which a thread
122    belongs.
123
124  - topology_logical_package_id():
125
126    The logical package ID to which a thread belongs.
127
128  - topology_physical_package_id():
129
130    The physical package ID to which a thread belongs.
131
132  - topology_core_id();
133
134    The ID of the core to which a thread belongs. It is also printed in /proc/cpuinfo
135    "core_id."
136
137
138
139System topology examples
140========================
141
142.. note::
143  The alternative Linux CPU enumeration depends on how the BIOS enumerates the
144  threads. Many BIOSes enumerate all threads 0 first and then all threads 1.
145  That has the "advantage" that the logical Linux CPU numbers of threads 0 stay
146  the same whether threads are enabled or not. That's merely an implementation
147  detail and has no practical impact.
148
1491) Single Package, Single Core::
150
151   [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
152
1532) Single Package, Dual Core
154
155   a) One thread per core::
156
157	[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
158		    -> [core 1] -> [thread 0] -> Linux CPU 1
159
160   b) Two threads per core::
161
162	[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
163				-> [thread 1] -> Linux CPU 1
164		    -> [core 1] -> [thread 0] -> Linux CPU 2
165				-> [thread 1] -> Linux CPU 3
166
167      Alternative enumeration::
168
169	[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
170				-> [thread 1] -> Linux CPU 2
171		    -> [core 1] -> [thread 0] -> Linux CPU 1
172				-> [thread 1] -> Linux CPU 3
173
174      AMD nomenclature for CMT systems::
175
176	[node 0] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 0
177				     -> [Compute Unit Core 1] -> Linux CPU 1
178		 -> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 2
179				     -> [Compute Unit Core 1] -> Linux CPU 3
180
1814) Dual Package, Dual Core
182
183   a) One thread per core::
184
185	[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
186		    -> [core 1] -> [thread 0] -> Linux CPU 1
187
188	[package 1] -> [core 0] -> [thread 0] -> Linux CPU 2
189		    -> [core 1] -> [thread 0] -> Linux CPU 3
190
191   b) Two threads per core::
192
193	[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
194				-> [thread 1] -> Linux CPU 1
195		    -> [core 1] -> [thread 0] -> Linux CPU 2
196				-> [thread 1] -> Linux CPU 3
197
198	[package 1] -> [core 0] -> [thread 0] -> Linux CPU 4
199				-> [thread 1] -> Linux CPU 5
200		    -> [core 1] -> [thread 0] -> Linux CPU 6
201				-> [thread 1] -> Linux CPU 7
202
203      Alternative enumeration::
204
205	[package 0] -> [core 0] -> [thread 0] -> Linux CPU 0
206				-> [thread 1] -> Linux CPU 4
207		    -> [core 1] -> [thread 0] -> Linux CPU 1
208				-> [thread 1] -> Linux CPU 5
209
210	[package 1] -> [core 0] -> [thread 0] -> Linux CPU 2
211				-> [thread 1] -> Linux CPU 6
212		    -> [core 1] -> [thread 0] -> Linux CPU 3
213				-> [thread 1] -> Linux CPU 7
214
215      AMD nomenclature for CMT systems::
216
217	[node 0] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 0
218				     -> [Compute Unit Core 1] -> Linux CPU 1
219		 -> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 2
220				     -> [Compute Unit Core 1] -> Linux CPU 3
221
222	[node 1] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 4
223				     -> [Compute Unit Core 1] -> Linux CPU 5
224		 -> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 6
225				     -> [Compute Unit Core 1] -> Linux CPU 7
226