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 - cpuinfo_x86.x86_max_cores: 51 52 The number of cores in a package. This information is retrieved via CPUID. 53 54 - cpuinfo_x86.x86_max_dies: 55 56 The number of dies in a package. This information is retrieved via CPUID. 57 58 - cpuinfo_x86.cpu_die_id: 59 60 The physical ID of the die. This information is retrieved via CPUID. 61 62 - cpuinfo_x86.phys_proc_id: 63 64 The physical ID of the package. This information is retrieved via CPUID 65 and deduced from the APIC IDs of the cores in the package. 66 67 Modern systems use this value for the socket. There may be multiple 68 packages within a socket. This value may differ from cpu_die_id. 69 70 - cpuinfo_x86.logical_proc_id: 71 72 The logical ID of the package. As we do not trust BIOSes to enumerate the 73 packages in a consistent way, we introduced the concept of logical package 74 ID so we can sanely calculate the number of maximum possible packages in 75 the system and have the packages enumerated linearly. 76 77 - topology_max_packages(): 78 79 The maximum possible number of packages in the system. Helpful for per 80 package facilities to preallocate per package information. 81 82 - cpu_llc_id: 83 84 A per-CPU variable containing: 85 86 - On Intel, the first APIC ID of the list of CPUs sharing the Last Level 87 Cache 88 89 - On AMD, the Node ID or Core Complex ID containing the Last Level 90 Cache. In general, it is a number identifying an LLC uniquely on the 91 system. 92 93Cores 94===== 95A core consists of 1 or more threads. It does not matter whether the threads 96are SMT- or CMT-type threads. 97 98AMDs nomenclature for a CMT core is "Compute Unit". The kernel always uses 99"core". 100 101Core-related topology information in the kernel: 102 103 - smp_num_siblings: 104 105 The number of threads in a core. The number of threads in a package can be 106 calculated by:: 107 108 threads_per_package = cpuinfo_x86.x86_max_cores * smp_num_siblings 109 110 111Threads 112======= 113A thread is a single scheduling unit. It's the equivalent to a logical Linux 114CPU. 115 116AMDs nomenclature for CMT threads is "Compute Unit Core". The kernel always 117uses "thread". 118 119Thread-related topology information in the kernel: 120 121 - topology_core_cpumask(): 122 123 The cpumask contains all online threads in the package to which a thread 124 belongs. 125 126 The number of online threads is also printed in /proc/cpuinfo "siblings." 127 128 - topology_sibling_cpumask(): 129 130 The cpumask contains all online threads in the core to which a thread 131 belongs. 132 133 - topology_logical_package_id(): 134 135 The logical package ID to which a thread belongs. 136 137 - topology_physical_package_id(): 138 139 The physical package ID to which a thread belongs. 140 141 - topology_core_id(); 142 143 The ID of the core to which a thread belongs. It is also printed in /proc/cpuinfo 144 "core_id." 145 146 147 148System topology examples 149======================== 150 151.. note:: 152 The alternative Linux CPU enumeration depends on how the BIOS enumerates the 153 threads. Many BIOSes enumerate all threads 0 first and then all threads 1. 154 That has the "advantage" that the logical Linux CPU numbers of threads 0 stay 155 the same whether threads are enabled or not. That's merely an implementation 156 detail and has no practical impact. 157 1581) Single Package, Single Core:: 159 160 [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 161 1622) Single Package, Dual Core 163 164 a) One thread per core:: 165 166 [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 167 -> [core 1] -> [thread 0] -> Linux CPU 1 168 169 b) Two threads per core:: 170 171 [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 172 -> [thread 1] -> Linux CPU 1 173 -> [core 1] -> [thread 0] -> Linux CPU 2 174 -> [thread 1] -> Linux CPU 3 175 176 Alternative enumeration:: 177 178 [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 179 -> [thread 1] -> Linux CPU 2 180 -> [core 1] -> [thread 0] -> Linux CPU 1 181 -> [thread 1] -> Linux CPU 3 182 183 AMD nomenclature for CMT systems:: 184 185 [node 0] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 0 186 -> [Compute Unit Core 1] -> Linux CPU 1 187 -> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 2 188 -> [Compute Unit Core 1] -> Linux CPU 3 189 1904) Dual Package, Dual Core 191 192 a) One thread per core:: 193 194 [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 195 -> [core 1] -> [thread 0] -> Linux CPU 1 196 197 [package 1] -> [core 0] -> [thread 0] -> Linux CPU 2 198 -> [core 1] -> [thread 0] -> Linux CPU 3 199 200 b) Two threads per core:: 201 202 [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 203 -> [thread 1] -> Linux CPU 1 204 -> [core 1] -> [thread 0] -> Linux CPU 2 205 -> [thread 1] -> Linux CPU 3 206 207 [package 1] -> [core 0] -> [thread 0] -> Linux CPU 4 208 -> [thread 1] -> Linux CPU 5 209 -> [core 1] -> [thread 0] -> Linux CPU 6 210 -> [thread 1] -> Linux CPU 7 211 212 Alternative enumeration:: 213 214 [package 0] -> [core 0] -> [thread 0] -> Linux CPU 0 215 -> [thread 1] -> Linux CPU 4 216 -> [core 1] -> [thread 0] -> Linux CPU 1 217 -> [thread 1] -> Linux CPU 5 218 219 [package 1] -> [core 0] -> [thread 0] -> Linux CPU 2 220 -> [thread 1] -> Linux CPU 6 221 -> [core 1] -> [thread 0] -> Linux CPU 3 222 -> [thread 1] -> Linux CPU 7 223 224 AMD nomenclature for CMT systems:: 225 226 [node 0] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 0 227 -> [Compute Unit Core 1] -> Linux CPU 1 228 -> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 2 229 -> [Compute Unit Core 1] -> Linux CPU 3 230 231 [node 1] -> [Compute Unit 0] -> [Compute Unit Core 0] -> Linux CPU 4 232 -> [Compute Unit Core 1] -> Linux CPU 5 233 -> [Compute Unit 1] -> [Compute Unit Core 0] -> Linux CPU 6 234 -> [Compute Unit Core 1] -> Linux CPU 7 235