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 44Modern systems may also use the term 'Die' for package. 45 46AMD nomenclature for package is 'Node'. 47 48Package-related topology information in the kernel: 49 50 - topology_num_threads_per_package() 51 52 The number of threads in a package. 53 54 - topology_num_cores_per_package() 55 56 The number of cores in a package. 57 58 - topology_max_dies_per_package() 59 60 The maximum number of dies in a package. 61 62 - cpuinfo_x86.topo.die_id: 63 64 The physical ID of the die. 65 66 - cpuinfo_x86.topo.pkg_id: 67 68 The physical ID of the package. This information is retrieved via CPUID 69 and deduced from the APIC IDs of the cores in the package. 70 71 Modern systems use this value for the socket. There may be multiple 72 packages within a socket. This value may differ from topo.die_id. 73 74 - cpuinfo_x86.topo.logical_pkg_id: 75 76 The logical ID of the package. As we do not trust BIOSes to enumerate the 77 packages in a consistent way, we introduced the concept of logical package 78 ID so we can sanely calculate the number of maximum possible packages in 79 the system and have the packages enumerated linearly. 80 81 - topology_max_packages(): 82 83 The maximum possible number of packages in the system. Helpful for per 84 package facilities to preallocate per package information. 85 86 - cpuinfo_x86.topo.llc_id: 87 88 - On Intel, the first APIC ID of the list of CPUs sharing the Last Level 89 Cache 90 91 - On AMD, the Node ID or Core Complex ID containing the Last Level 92 Cache. In general, it is a number identifying an LLC uniquely on the 93 system. 94 95Cores 96===== 97A core consists of 1 or more threads. It does not matter whether the threads 98are SMT- or CMT-type threads. 99 100AMDs nomenclature for a CMT core is "Compute Unit". The kernel always uses 101"core". 102 103Threads 104======= 105A thread is a single scheduling unit. It's the equivalent to a logical Linux 106CPU. 107 108AMDs nomenclature for CMT threads is "Compute Unit Core". The kernel always 109uses "thread". 110 111Thread-related topology information in the kernel: 112 113 - topology_core_cpumask(): 114 115 The cpumask contains all online threads in the package to which a thread 116 belongs. 117 118 The number of online threads is also printed in /proc/cpuinfo "siblings." 119 120 - topology_sibling_cpumask(): 121 122 The cpumask contains all online threads in the core to which a thread 123 belongs. 124 125 - topology_logical_package_id(): 126 127 The logical package ID to which a thread belongs. 128 129 - topology_physical_package_id(): 130 131 The physical package ID to which a thread belongs. 132 133 - topology_core_id(); 134 135 The ID of the core to which a thread belongs. It is also printed in /proc/cpuinfo 136 "core_id." 137 138 - topology_logical_core_id(); 139 140 The logical core ID to which a thread belongs. 141 142 143 144System topology examples 145======================== 146 147.. note:: 148 The alternative Linux CPU enumeration depends on how the BIOS enumerates the 149 threads. Many BIOSes enumerate all threads 0 first and then all threads 1. 150 That has the "advantage" that the logical Linux CPU numbers of threads 0 stay 151 the same whether threads are enabled or not. That's merely an implementation 152 detail and has no practical impact. 153 1541) Single Package, Single Core:: 155 156 [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 157 1582) Single Package, Dual Core 159 160 a) One thread per core:: 161 162 [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 163 -> [core 1] -> [thread 0] -> Linux CPU 1 164 165 b) Two threads per core:: 166 167 [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 168 -> [thread 1] -> Linux CPU 1 169 -> [core 1] -> [thread 0] -> Linux CPU 2 170 -> [thread 1] -> Linux CPU 3 171 172 Alternative enumeration:: 173 174 [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 175 -> [thread 1] -> Linux CPU 2 176 -> [core 1] -> [thread 0] -> Linux CPU 1 177 -> [thread 1] -> Linux CPU 3 178 179 AMD nomenclature for CMT systems:: 180 181 [node 0] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 0 182 -> [Compute Unit Core 1] -> Linux CPU 1 183 -> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 2 184 -> [Compute Unit Core 1] -> Linux CPU 3 185 1864) Dual Package, Dual Core 187 188 a) One thread per core:: 189 190 [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 191 -> [core 1] -> [thread 0] -> Linux CPU 1 192 193 [package 1] -> [core 0] -> [thread 0] -> Linux CPU 2 194 -> [core 1] -> [thread 0] -> Linux CPU 3 195 196 b) Two threads per core:: 197 198 [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 199 -> [thread 1] -> Linux CPU 1 200 -> [core 1] -> [thread 0] -> Linux CPU 2 201 -> [thread 1] -> Linux CPU 3 202 203 [package 1] -> [core 0] -> [thread 0] -> Linux CPU 4 204 -> [thread 1] -> Linux CPU 5 205 -> [core 1] -> [thread 0] -> Linux CPU 6 206 -> [thread 1] -> Linux CPU 7 207 208 Alternative enumeration:: 209 210 [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 211 -> [thread 1] -> Linux CPU 4 212 -> [core 1] -> [thread 0] -> Linux CPU 1 213 -> [thread 1] -> Linux CPU 5 214 215 [package 1] -> [core 0] -> [thread 0] -> Linux CPU 2 216 -> [thread 1] -> Linux CPU 6 217 -> [core 1] -> [thread 0] -> Linux CPU 3 218 -> [thread 1] -> Linux CPU 7 219 220 AMD nomenclature for CMT systems:: 221 222 [node 0] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 0 223 -> [Compute Unit Core 1] -> Linux CPU 1 224 -> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 2 225 -> [Compute Unit Core 1] -> Linux CPU 3 226 227 [node 1] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 4 228 -> [Compute Unit Core 1] -> Linux CPU 5 229 -> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 6 230 -> [Compute Unit Core 1] -> Linux CPU 7 231