Documentation/core-api/this_cpu_ops.rst

8 this_cpu operations are a way of optimizing access to per cpu
11 the cpu permanently stored the beginning of the per cpu	area for a
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.
32 synchronization is not necessary since we are dealing with per cpu
37 Please note that accesses by remote processors to a per cpu area are
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
109 Note that these operations can only be used on per cpu data that is
112 per cpu counters is correctly incremented. However, there is no
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
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
138 available. Instead, the offset of the local per cpu area is simply
139 added to the per cpu offset.
143 disabled. The pointer is then used to access local per cpu data in a
145 no longer useful since it may no longer point to per cpu data of the
148 The special cases where it makes sense to obtain a per-CPU pointer in
150 to handle cases where two different CPUs are accessing the same per cpu
153 of counters as a pair of per-CPU variables, and rcu_read_lock_nmisafe()
158 Per cpu variables and offsets
161 Per cpu variables have *offsets* to the beginning of the per cpu
164 added to a base pointer of a per cpu area of a processor in order to
167 Therefore the use of x or &x outside of the context of per cpu
175 In the context of per cpu operations the above implies that x is a per
182 &x and hence p is the *offset* of a per cpu variable. this_cpu_ptr()
183 takes the offset of a per cpu variable which makes this look a bit
187 Operations on a field of a per cpu structure
231 these per cpu local operations. In that case the operation must be
240 preemption. If a per cpu variable is not used in an interrupt context
281 Remote access to per cpu data
284 Per cpu data structures are designed to be used by one cpu exclusively.
288 There are special cases where you might need to access per cpu data
297 the remote CPU and perform the update to its per cpu area.
299 To access per-cpu data structure remotely, typically the per_cpu_ptr()
318 per cpu data. Write accesses can cause unique problems due to the
322 the following scenario that occurs because two per cpu variables
341 of a remote write to the per cpu area of another processor.
346 missing local cache line of a per cpu area, its performance and hence