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Linux/Documentation/this_cpu_ops.txt

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

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