1this_cpu operations 2------------------- 3 4this_cpu operations are a way of optimizing access to per cpu 5variables associated with the *currently* executing processor. This is 6done through the use of segment registers (or a dedicated register where 7the cpu permanently stored the beginning of the per cpu area for a 8specific processor). 9 10this_cpu operations add a per cpu variable offset to the processor 11specific per cpu base and encode that operation in the instruction 12operating on the per cpu variable. 13 14This means that there are no atomicity issues between the calculation of 15the offset and the operation on the data. Therefore it is not 16necessary to disable preemption or interrupts to ensure that the 17processor is not changed between the calculation of the address and 18the operation on the data. 19 20Read-modify-write operations are of particular interest. Frequently 21processors have special lower latency instructions that can operate 22without the typical synchronization overhead, but still provide some 23sort of relaxed atomicity guarantees. The x86, for example, can execute 24RMW (Read Modify Write) instructions like inc/dec/cmpxchg without the 25lock prefix and the associated latency penalty. 26 27Access to the variable without the lock prefix is not synchronized but 28synchronization is not necessary since we are dealing with per cpu 29data specific to the currently executing processor. Only the current 30processor should be accessing that variable and therefore there are no 31concurrency issues with other processors in the system. 32 33Please note that accesses by remote processors to a per cpu area are 34exceptional situations and may impact performance and/or correctness 35(remote write operations) of local RMW operations via this_cpu_*. 36 37The main use of the this_cpu operations has been to optimize counter 38operations. 39 40The following this_cpu() operations with implied preemption protection 41are defined. These operations can be used without worrying about 42preemption and interrupts. 43 44 this_cpu_read(pcp) 45 this_cpu_write(pcp, val) 46 this_cpu_add(pcp, val) 47 this_cpu_and(pcp, val) 48 this_cpu_or(pcp, val) 49 this_cpu_add_return(pcp, val) 50 this_cpu_xchg(pcp, nval) 51 this_cpu_cmpxchg(pcp, oval, nval) 52 this_cpu_cmpxchg_double(pcp1, pcp2, oval1, oval2, nval1, nval2) 53 this_cpu_sub(pcp, val) 54 this_cpu_inc(pcp) 55 this_cpu_dec(pcp) 56 this_cpu_sub_return(pcp, val) 57 this_cpu_inc_return(pcp) 58 this_cpu_dec_return(pcp) 59 60 61Inner working of this_cpu operations 62------------------------------------ 63 64On x86 the fs: or the gs: segment registers contain the base of the 65per cpu area. It is then possible to simply use the segment override 66to relocate a per cpu relative address to the proper per cpu area for 67the processor. So the relocation to the per cpu base is encoded in the 68instruction via a segment register prefix. 69 70For example: 71 72 DEFINE_PER_CPU(int, x); 73 int z; 74 75 z = this_cpu_read(x); 76 77results in a single instruction 78 79 mov ax, gs:[x] 80 81instead of a sequence of calculation of the address and then a fetch 82from that address which occurs with the per cpu operations. Before 83this_cpu_ops such sequence also required preempt disable/enable to 84prevent the kernel from moving the thread to a different processor 85while the calculation is performed. 86 87Consider the following this_cpu operation: 88 89 this_cpu_inc(x) 90 91The above results in the following single instruction (no lock prefix!) 92 93 inc gs:[x] 94 95instead of the following operations required if there is no segment 96register: 97 98 int *y; 99 int cpu; 100 101 cpu = get_cpu(); 102 y = per_cpu_ptr(&x, cpu); 103 (*y)++; 104 put_cpu(); 105 106Note that these operations can only be used on per cpu data that is 107reserved for a specific processor. Without disabling preemption in the 108surrounding code this_cpu_inc() will only guarantee that one of the 109per cpu counters is correctly incremented. However, there is no 110guarantee that the OS will not move the process directly before or 111after the this_cpu instruction is executed. In general this means that 112the value of the individual counters for each processor are 113meaningless. The sum of all the per cpu counters is the only value 114that is of interest. 115 116Per cpu variables are used for performance reasons. Bouncing cache 117lines can be avoided if multiple processors concurrently go through 118the same code paths. Since each processor has its own per cpu 119variables no concurrent cache line updates take place. The price that 120has to be paid for this optimization is the need to add up the per cpu 121counters when the value of a counter is needed. 122 123 124Special operations: 125------------------- 126 127 y = this_cpu_ptr(&x) 128 129Takes the offset of a per cpu variable (&x !) and returns the address 130of the per cpu variable that belongs to the currently executing 131processor. this_cpu_ptr avoids multiple steps that the common 132get_cpu/put_cpu sequence requires. No processor number is 133available. Instead, the offset of the local per cpu area is simply 134added to the per cpu offset. 135 136Note that this operation is usually used in a code segment when 137preemption has been disabled. The pointer is then used to 138access local per cpu data in a critical section. When preemption 139is re-enabled this pointer is usually no longer useful since it may 140no longer point to per cpu data of the current processor. 141 142 143Per cpu variables and offsets 144----------------------------- 145 146Per cpu variables have *offsets* to the beginning of the per cpu 147area. They do not have addresses although they look like that in the 148code. Offsets cannot be directly dereferenced. The offset must be 149added to a base pointer of a per cpu area of a processor in order to 150form a valid address. 151 152Therefore the use of x or &x outside of the context of per cpu 153operations is invalid and will generally be treated like a NULL 154pointer dereference. 155 156 DEFINE_PER_CPU(int, x); 157 158In the context of per cpu operations the above implies that x is a per 159cpu variable. Most this_cpu operations take a cpu variable. 160 161 int __percpu *p = &x; 162 163&x and hence p is the *offset* of a per cpu variable. this_cpu_ptr() 164takes the offset of a per cpu variable which makes this look a bit 165strange. 166 167 168Operations on a field of a per cpu structure 169-------------------------------------------- 170 171Let's say we have a percpu structure 172 173 struct s { 174 int n,m; 175 }; 176 177 DEFINE_PER_CPU(struct s, p); 178 179 180Operations on these fields are straightforward 181 182 this_cpu_inc(p.m) 183 184 z = this_cpu_cmpxchg(p.m, 0, 1); 185 186 187If we have an offset to struct s: 188 189 struct s __percpu *ps = &p; 190 191 this_cpu_dec(ps->m); 192 193 z = this_cpu_inc_return(ps->n); 194 195 196The calculation of the pointer may require the use of this_cpu_ptr() 197if we do not make use of this_cpu ops later to manipulate fields: 198 199 struct s *pp; 200 201 pp = this_cpu_ptr(&p); 202 203 pp->m--; 204 205 z = pp->n++; 206 207 208Variants of this_cpu ops 209------------------------- 210 211this_cpu ops are interrupt safe. Some architectures do not support 212these per cpu local operations. In that case the operation must be 213replaced by code that disables interrupts, then does the operations 214that are guaranteed to be atomic and then re-enable interrupts. Doing 215so is expensive. If there are other reasons why the scheduler cannot 216change the processor we are executing on then there is no reason to 217disable interrupts. For that purpose the following __this_cpu operations 218are provided. 219 220These operations have no guarantee against concurrent interrupts or 221preemption. If a per cpu variable is not used in an interrupt context 222and the scheduler cannot preempt, then they are safe. If any interrupts 223still occur while an operation is in progress and if the interrupt too 224modifies the variable, then RMW actions can not be guaranteed to be 225safe. 226 227 __this_cpu_read(pcp) 228 __this_cpu_write(pcp, val) 229 __this_cpu_add(pcp, val) 230 __this_cpu_and(pcp, val) 231 __this_cpu_or(pcp, val) 232 __this_cpu_add_return(pcp, val) 233 __this_cpu_xchg(pcp, nval) 234 __this_cpu_cmpxchg(pcp, oval, nval) 235 __this_cpu_cmpxchg_double(pcp1, pcp2, oval1, oval2, nval1, nval2) 236 __this_cpu_sub(pcp, val) 237 __this_cpu_inc(pcp) 238 __this_cpu_dec(pcp) 239 __this_cpu_sub_return(pcp, val) 240 __this_cpu_inc_return(pcp) 241 __this_cpu_dec_return(pcp) 242 243 244Will increment x and will not fall-back to code that disables 245interrupts on platforms that cannot accomplish atomicity through 246address relocation and a Read-Modify-Write operation in the same 247instruction. 248 249 250&this_cpu_ptr(pp)->n vs this_cpu_ptr(&pp->n) 251-------------------------------------------- 252 253The first operation takes the offset and forms an address and then 254adds the offset of the n field. This may result in two add 255instructions emitted by the compiler. 256 257The second one first adds the two offsets and then does the 258relocation. IMHO the second form looks cleaner and has an easier time 259with (). The second form also is consistent with the way 260this_cpu_read() and friends are used. 261 262 263Remote access to per cpu data 264------------------------------ 265 266Per cpu data structures are designed to be used by one cpu exclusively. 267If you use the variables as intended, this_cpu_ops() are guaranteed to 268be "atomic" as no other CPU has access to these data structures. 269 270There are special cases where you might need to access per cpu data 271structures remotely. It is usually safe to do a remote read access 272and that is frequently done to summarize counters. Remote write access 273something which could be problematic because this_cpu ops do not 274have lock semantics. A remote write may interfere with a this_cpu 275RMW operation. 276 277Remote write accesses to percpu data structures are highly discouraged 278unless absolutely necessary. Please consider using an IPI to wake up 279the remote CPU and perform the update to its per cpu area. 280 281To access per-cpu data structure remotely, typically the per_cpu_ptr() 282function is used: 283 284 285 DEFINE_PER_CPU(struct data, datap); 286 287 struct data *p = per_cpu_ptr(&datap, cpu); 288 289This makes it explicit that we are getting ready to access a percpu 290area remotely. 291 292You can also do the following to convert the datap offset to an address 293 294 struct data *p = this_cpu_ptr(&datap); 295 296but, passing of pointers calculated via this_cpu_ptr to other cpus is 297unusual and should be avoided. 298 299Remote access are typically only for reading the status of another cpus 300per cpu data. Write accesses can cause unique problems due to the 301relaxed synchronization requirements for this_cpu operations. 302 303One example that illustrates some concerns with write operations is 304the following scenario that occurs because two per cpu variables 305share a cache-line but the relaxed synchronization is applied to 306only one process updating the cache-line. 307 308Consider the following example 309 310 311 struct test { 312 atomic_t a; 313 int b; 314 }; 315 316 DEFINE_PER_CPU(struct test, onecacheline); 317 318There is some concern about what would happen if the field 'a' is updated 319remotely from one processor and the local processor would use this_cpu ops 320to update field b. Care should be taken that such simultaneous accesses to 321data within the same cache line are avoided. Also costly synchronization 322may be necessary. IPIs are generally recommended in such scenarios instead 323of a remote write to the per cpu area of another processor. 324 325Even in cases where the remote writes are rare, please bear in 326mind that a remote write will evict the cache line from the processor 327that most likely will access it. If the processor wakes up and finds a 328missing local cache line of a per cpu area, its performance and hence 329the wake up times will be affected. 330 331Christoph Lameter, August 4th, 2014 332Pranith Kumar, Aug 2nd, 2014 333