===================

  this_cpu operations

  ===================
  
  :Author: Christoph Lameter, August 4th, 2014
  :Author: Pranith Kumar, Aug 2nd, 2014

  
  this_cpu operations are a way of optimizing access to per cpu

  variables associated with the *currently* executing processor. This is
  done 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).

  this_cpu operations add a per cpu variable offset to the processor
  specific per cpu base and encode that operation in the instruction

  operating on the per cpu variable.

  This means that there are no atomicity issues between the calculation of

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

  necessary to disable preemption 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 guarantees. The x86, for example, can execute
  RMW (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.

  Please note that accesses by remote processors to a per cpu area are
  exceptional situations and may impact performance and/or correctness
  (remote write operations) of local RMW operations via this_cpu_*.
  
  The main use of the this_cpu operations has been to optimize counter
  operations.
  
  The following this_cpu() operations with implied preemption protection
  are defined. These operations can be used without worrying about

  preemption and interrupts::

  	this_cpu_read(pcp)
  	this_cpu_write(pcp, val)
  	this_cpu_add(pcp, val)
  	this_cpu_and(pcp, val)
  	this_cpu_or(pcp, val)
  	this_cpu_add_return(pcp, val)
  	this_cpu_xchg(pcp, nval)
  	this_cpu_cmpxchg(pcp, oval, nval)
  	this_cpu_cmpxchg_double(pcp1, pcp2, oval1, oval2, nval1, nval2)
  	this_cpu_sub(pcp, val)
  	this_cpu_inc(pcp)
  	this_cpu_dec(pcp)
  	this_cpu_sub_return(pcp, val)
  	this_cpu_inc_return(pcp)
  	this_cpu_dec_return(pcp)
  
  
  Inner working of this_cpu operations
  ------------------------------------

  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 per cpu 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.

  Consider the following this_cpu operation::

  
  	this_cpu_inc(x)

  The above 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 per cpu 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

  per cpu 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 cache line 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 a 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 per cpu offset.

  Note that this operation is usually used in a code segment when
  preemption has been disabled. The pointer is then used to
  access local per cpu data in a critical section. When preemption
  is re-enabled this pointer is usually no longer useful since it may
  no longer point to per cpu data of the current processor.

  
  
  Per cpu variables and offsets
  -----------------------------

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

  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 per cpu 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.

  ::

  	DEFINE_PER_CPU(int, x);

  In the context of per cpu operations the above implies that x is a per
  cpu variable. Most this_cpu operations take a cpu variable.

  ::

  	int __percpu *p = &x;

  &x and hence p is the *offset* of 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;

  	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 architectures 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 re-enable 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 following __this_cpu operations
  are provided.
  
  These operations have no guarantee against concurrent interrupts or
  preemption. If a per cpu variable is not used in an interrupt context
  and the scheduler cannot preempt, then they are safe. If any interrupts
  still occur while an operation is in progress and if the interrupt too
  modifies the variable, then RMW actions can not be guaranteed to be

  safe::

  	__this_cpu_read(pcp)
  	__this_cpu_write(pcp, val)
  	__this_cpu_add(pcp, val)
  	__this_cpu_and(pcp, val)
  	__this_cpu_or(pcp, val)
  	__this_cpu_add_return(pcp, val)
  	__this_cpu_xchg(pcp, nval)
  	__this_cpu_cmpxchg(pcp, oval, nval)
  	__this_cpu_cmpxchg_double(pcp1, pcp2, oval1, oval2, nval1, nval2)
  	__this_cpu_sub(pcp, val)
  	__this_cpu_inc(pcp)
  	__this_cpu_dec(pcp)
  	__this_cpu_sub_return(pcp, val)
  	__this_cpu_inc_return(pcp)
  	__this_cpu_dec_return(pcp)
  
  
  Will increment x and will not fall-back 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. This may result in two add
  instructions emitted by the compiler.

  
  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.

  Remote access to per cpu data
  ------------------------------
  
  Per cpu data structures are designed to be used by one cpu exclusively.
  If you use the variables as intended, this_cpu_ops() are guaranteed to
  be "atomic" as no other CPU has access to these data structures.
  
  There are special cases where you might need to access per cpu data
  structures remotely. It is usually safe to do a remote read access
  and that is frequently done to summarize counters. Remote write access
  something which could be problematic because this_cpu ops do not
  have lock semantics. A remote write may interfere with a this_cpu
  RMW operation.
  
  Remote write accesses to percpu data structures are highly discouraged
  unless absolutely necessary. Please consider using an IPI to wake up
  the remote CPU and perform the update to its per cpu area.
  
  To access per-cpu data structure remotely, typically the per_cpu_ptr()

  function is used::

  
  
  	DEFINE_PER_CPU(struct data, datap);
  
  	struct data *p = per_cpu_ptr(&datap, cpu);
  
  This makes it explicit that we are getting ready to access a percpu
  area remotely.

  You can also do the following to convert the datap offset to an address::

  
  	struct data *p = this_cpu_ptr(&datap);
  
  but, passing of pointers calculated via this_cpu_ptr to other cpus is
  unusual and should be avoided.
  
  Remote access are typically only for reading the status of another cpus
  per cpu data. Write accesses can cause unique problems due to the
  relaxed synchronization requirements for this_cpu operations.
  
  One example that illustrates some concerns with write operations is
  the following scenario that occurs because two per cpu variables
  share a cache-line but the relaxed synchronization is applied to
  only one process updating the cache-line.

  Consider the following example::

  
  
  	struct test {
  		atomic_t a;
  		int b;
  	};
  
  	DEFINE_PER_CPU(struct test, onecacheline);
  
  There is some concern about what would happen if the field 'a' is updated
  remotely from one processor and the local processor would use this_cpu ops
  to update field b. Care should be taken that such simultaneous accesses to
  data within the same cache line are avoided. Also costly synchronization
  may be necessary. IPIs are generally recommended in such scenarios instead
  of a remote write to the per cpu area of another processor.
  
  Even in cases where the remote writes are rare, please bear in
  mind that a remote write will evict the cache line from the processor
  that most likely will access it. If the processor wakes up and finds a
  missing local cache line of a per cpu area, its performance and hence
  the wake up times will be affected.