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Merge branch 'for-3.10' of git://git.kernel.org/pub/scm/linux/kernel/…
…git/tj/percpu Pull percpu patch from Tejun Heo: "A puny pull request for percpu. We were expecting more cleanup patches but didn't happen this time, so just a single patch adding documentation from Christoph." * 'for-3.10' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/percpu: percpu: add documentation on this_cpu operations
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this_cpu operations | ||
------------------- | ||
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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). | ||
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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. | ||
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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. | ||
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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. | ||
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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. | ||
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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. | ||
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For example: | ||
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DEFINE_PER_CPU(int, x); | ||
int z; | ||
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z = this_cpu_read(x); | ||
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results in a single instruction | ||
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mov ax, gs:[x] | ||
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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. | ||
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The main use of the this_cpu operations has been to optimize counter | ||
operations. | ||
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this_cpu_inc(x) | ||
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results in the following single instruction (no lock prefix!) | ||
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inc gs:[x] | ||
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instead of the following operations required if there is no segment | ||
register. | ||
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int *y; | ||
int cpu; | ||
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cpu = get_cpu(); | ||
y = per_cpu_ptr(&x, cpu); | ||
(*y)++; | ||
put_cpu(); | ||
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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. | ||
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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. | ||
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Special operations: | ||
------------------- | ||
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y = this_cpu_ptr(&x) | ||
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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. | ||
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Per cpu variables and offsets | ||
----------------------------- | ||
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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. | ||
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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. | ||
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In the context of per cpu operations | ||
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x is a per cpu variable. Most this_cpu operations take a cpu | ||
variable. | ||
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&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. | ||
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Operations on a field of a per cpu structure | ||
-------------------------------------------- | ||
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Let's say we have a percpu structure | ||
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struct s { | ||
int n,m; | ||
}; | ||
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DEFINE_PER_CPU(struct s, p); | ||
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Operations on these fields are straightforward | ||
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this_cpu_inc(p.m) | ||
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z = this_cpu_cmpxchg(p.m, 0, 1); | ||
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If we have an offset to struct s: | ||
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struct s __percpu *ps = &p; | ||
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z = this_cpu_dec(ps->m); | ||
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z = this_cpu_inc_return(ps->n); | ||
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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: | ||
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struct s *pp; | ||
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pp = this_cpu_ptr(&p); | ||
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pp->m--; | ||
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z = pp->n++; | ||
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Variants of this_cpu ops | ||
------------------------- | ||
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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. | ||
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__this_cpu_inc(x); | ||
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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. | ||
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&this_cpu_ptr(pp)->n vs this_cpu_ptr(&pp->n) | ||
-------------------------------------------- | ||
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The first operation takes the offset and forms an address and then | ||
adds the offset of the n field. | ||
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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. | ||
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Christoph Lameter, April 3rd, 2013 |