Documentation/core-api/this_cpu_ops.rst

8 this_cpu operations are a way of optimizing access to per cpu
9 variables associated with the *currently* executing processor. This is
11 the cpu permanently stored the beginning of the per cpu	area for a
12 specific processor).
14 this_cpu operations add a per cpu variable offset to the processor
15 specific per cpu base and encode that operation in the instruction
16 operating on the per cpu variable.
21 processor is not changed between the calculation of the address and
24 Read-modify-write operations are of particular interest. Frequently
32 synchronization is not necessary since we are dealing with per cpu
33 data specific to the currently executing processor. Only the current
34 processor should be accessing that variable and therefore there are no
37 Please note that accesses by remote processors to a per cpu area are
65 ------------------------------------
68 per cpu area. It is then possible to simply use the segment override
69 to relocate a per cpu relative address to the proper per cpu area for
70 the processor. So the relocation to the per cpu base is encoded in the
85 from that address which occurs with the per cpu operations. Before
87 prevent the kernel from moving the thread to a different processor
109 Note that these operations can only be used on per cpu data that is
110 reserved for a specific processor. Without disabling preemption in the
112 per cpu counters is correctly incremented. However, there is no
115 the value of the individual counters for each processor are
116 meaningless. The sum of all the per cpu counters is the only value
119 Per cpu variables are used for performance reasons. Bouncing cache
121 the same code paths.  Since each processor has its own per cpu
123 has to be paid for this optimization is the need to add up the per cpu
128 ------------------
134 Takes the offset of a per cpu variable (&x !) and returns the address
135 of the per cpu variable that belongs to the currently executing
136 processor.  this_cpu_ptr avoids multiple steps that the common
137 get_cpu/put_cpu sequence requires. No processor number is
138 available. Instead, the offset of the local per cpu area is simply
139 added to the per cpu offset.
143 access local per cpu data in a critical section. When preemption
144 is re-enabled this pointer is usually no longer useful since it may
145 no longer point to per cpu data of the current processor.
148 Per cpu variables and offsets
149 -----------------------------
151 Per cpu variables have *offsets* to the beginning of the per cpu
154 added to a base pointer of a per cpu area of a processor in order to
157 Therefore the use of x or &x outside of the context of per cpu
165 In the context of per cpu operations the above implies that x is a per
172 &x and hence p is the *offset* of a per cpu variable. this_cpu_ptr()
173 takes the offset of a per cpu variable which makes this look a bit
177 Operations on a field of a per cpu structure
178 --------------------------------------------
200 	this_cpu_dec(ps->m);
202 	z = this_cpu_inc_return(ps->n);
212 	pp->m--;
214 	z = pp->n++;
218 ------------------------
221 these per cpu local operations. In that case the operation must be
223 that are guaranteed to be atomic and then re-enable interrupts. Doing
225 change the processor we are executing on then there is no reason to
230 preemption. If a per cpu variable is not used in an interrupt context
252 Will increment x and will not fall-back to code that disables
254 address relocation and a Read-Modify-Write operation in the same
258 &this_cpu_ptr(pp)->n vs this_cpu_ptr(&pp->n)
259 --------------------------------------------
271 Remote access to per cpu data
272 ------------------------------
274 Per cpu data structures are designed to be used by one cpu exclusively.
278 There are special cases where you might need to access per cpu data
287 the remote CPU and perform the update to its per cpu area.
289 To access per-cpu data structure remotely, typically the per_cpu_ptr()
308 per cpu data. Write accesses can cause unique problems due to the
312 the following scenario that occurs because two per cpu variables
313 share a cache-line but the relaxed synchronization is applied to
314 only one process updating the cache-line.
327 remotely from one processor and the local processor would use this_cpu ops
331 of a remote write to the per cpu area of another processor.
334 mind that a remote write will evict the cache line from the processor
335 that most likely will access it. If the processor wakes up and finds a
336 missing local cache line of a per cpu area, its performance and hence