percpu: add documentation on this_cpu operations

this_cpu operations

-------------------

this_cpu operations are a way of optimizing access to per cpu

variables associated with the *currently* executing processor through

the use of segment registers (or a dedicated register where the cpu

permanently stored the beginning of the per cpu area for a specific

processor).

The this_cpu operations add a per cpu variable offset to the processor

specific percpu base and encode that operation in the instruction

operating on the per cpu variable.

This means there are no atomicity issues between the calculation of

the offset and the operation on the data. Therefore it is not

necessary to disable preempt or interrupts to ensure that the

processor is not changed between the calculation of the address and

the operation on the data.

Read-modify-write operations are of particular interest. Frequently

processors have special lower latency instructions that can operate

without the typical synchronization overhead but still provide some

sort of relaxed atomicity guarantee. The x86 for example can execute

RMV (Read Modify Write) instructions like inc/dec/cmpxchg without the

lock prefix and the associated latency penalty.

Access to the variable without the lock prefix is not synchronized but

synchronization is not necessary since we are dealing with per cpu

data specific to the currently executing processor. Only the current

processor should be accessing that variable and therefore there are no

concurrency issues with other processors in the system.

On x86 the fs: or the gs: segment registers contain the base of the

per cpu area. It is then possible to simply use the segment override

to relocate a per cpu relative address to the proper per cpu area for

the processor. So the relocation to the per cpu base is encoded in the

instruction via a segment register prefix.

For example:

	DEFINE_PER_CPU(int, x);

	int z;

	z = this_cpu_read(x);

results in a single instruction

	mov ax, gs:[x]

instead of a sequence of calculation of the address and then a fetch

from that address which occurs with the percpu operations. Before

this_cpu_ops such sequence also required preempt disable/enable to

prevent the kernel from moving the thread to a different processor

while the calculation is performed.

The main use of the this_cpu operations has been to optimize counter

operations.

	this_cpu_inc(x)

results in the following single instruction (no lock prefix!)

	inc gs:[x]

instead of the following operations required if there is no segment

register.

	int *y;

	int cpu;

	cpu = get_cpu();

	y = per_cpu_ptr(&x, cpu);

	(*y)++;

	put_cpu();

Note that these operations can only be used on percpu data that is

reserved for a specific processor. Without disabling preemption in the

surrounding code this_cpu_inc() will only guarantee that one of the

percpu counters is correctly incremented. However, there is no

guarantee that the OS will not move the process directly before or

after the this_cpu instruction is executed. In general this means that

the value of the individual counters for each processor are

meaningless. The sum of all the per cpu counters is the only value

that is of interest.

Per cpu variables are used for performance reasons. Bouncing cache

lines can be avoided if multiple processors concurrently go through

the same code paths.  Since each processor has its own per cpu

variables no concurrent cacheline updates take place. The price that

has to be paid for this optimization is the need to add up the per cpu

counters when the value of the counter is needed.

Special operations:

-------------------

	y = this_cpu_ptr(&x)

Takes the offset of a per cpu variable (&x !) and returns the address

of the per cpu variable that belongs to the currently executing

processor.  this_cpu_ptr avoids multiple steps that the common

get_cpu/put_cpu sequence requires. No processor number is

available. Instead the offset of the local per cpu area is simply

added to the percpu offset.

Per cpu variables and offsets

-----------------------------

Per cpu variables have *offsets* to the beginning of the percpu

area. They do not have addresses although they look like that in the

code. Offsets cannot be directly dereferenced. The offset must be

added to a base pointer of a percpu area of a processor in order to

form a valid address.

Therefore the use of x or &x outside of the context of per cpu

operations is invalid and will generally be treated like a NULL

pointer dereference.

In the context of per cpu operations

	x is a per cpu variable. Most this_cpu operations take a cpu

	variable.

	&x is the *offset* a per cpu variable. this_cpu_ptr() takes

	the offset of a per cpu variable which makes this look a bit

	strange.

Operations on a field of a per cpu structure

--------------------------------------------

Let's say we have a percpu structure

	struct s {

		int n,m;

	};

	DEFINE_PER_CPU(struct s, p);

Operations on these fields are straightforward

	this_cpu_inc(p.m)

	z = this_cpu_cmpxchg(p.m, 0, 1);

If we have an offset to struct s:

	struct s __percpu *ps = &p;

	z = this_cpu_dec(ps->m);

	z = this_cpu_inc_return(ps->n);

The calculation of the pointer may require the use of this_cpu_ptr()

if we do not make use of this_cpu ops later to manipulate fields:

	struct s *pp;

	pp = this_cpu_ptr(&p);

	pp->m--;

	z = pp->n++;

Variants of this_cpu ops

-------------------------

this_cpu ops are interrupt safe. Some architecture do not support

these per cpu local operations. In that case the operation must be

replaced by code that disables interrupts, then does the operations

that are guaranteed to be atomic and then reenable interrupts. Doing

so is expensive. If there are other reasons why the scheduler cannot

change the processor we are executing on then there is no reason to

disable interrupts. For that purpose the __this_cpu operations are

provided. For example.

	__this_cpu_inc(x);

Will increment x and will not fallback to code that disables

interrupts on platforms that cannot accomplish atomicity through

address relocation and a Read-Modify-Write operation in the same

instruction.

&this_cpu_ptr(pp)->n vs this_cpu_ptr(&pp->n)

--------------------------------------------

The first operation takes the offset and forms an address and then

adds the offset of the n field.

The second one first adds the two offsets and then does the

relocation.  IMHO the second form looks cleaner and has an easier time

with (). The second form also is consistent with the way

this_cpu_read() and friends are used.

Christoph Lameter, April 3rd, 2013