percpu: add documentation on this_cpu operations

Document the rationale and the way to use this_cpu operations.

V2: Improved after feedback from Randy Dunlap

v3: Further spelling fixes from Randy.  Paragraphs refilled to 75
    column.

tj: Added .txt file extension to the document.
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Documentation/this_cpu_ops.txt

+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