mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-27 03:10:12 +00:00
![Christoph Lameter](/assets/img/avatar_default.png)
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>
205 lines
6.4 KiB
Text
205 lines
6.4 KiB
Text
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
|