338 lines
11 KiB
ReStructuredText
338 lines
11 KiB
ReStructuredText
===================
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this_cpu operations
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===================
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:Author: Christoph Lameter, August 4th, 2014
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:Author: Pranith Kumar, Aug 2nd, 2014
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this_cpu operations are a way of optimizing access to per cpu
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variables associated with the *currently* executing processor. This is
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done through the use of segment registers (or a dedicated register where
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the cpu permanently stored the beginning of the per cpu area for a
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specific processor).
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this_cpu operations add a per cpu variable offset to the processor
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specific per cpu base and encode that operation in the instruction
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operating on the per cpu variable.
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This means that there are no atomicity issues between the calculation of
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the offset and the operation on the data. Therefore it is not
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necessary to disable preemption or interrupts to ensure that the
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processor is not changed between the calculation of the address and
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the operation on the data.
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Read-modify-write operations are of particular interest. Frequently
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processors have special lower latency instructions that can operate
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without the typical synchronization overhead, but still provide some
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sort of relaxed atomicity guarantees. The x86, for example, can execute
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RMW (Read Modify Write) instructions like inc/dec/cmpxchg without the
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lock prefix and the associated latency penalty.
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Access to the variable without the lock prefix is not synchronized but
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synchronization is not necessary since we are dealing with per cpu
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data specific to the currently executing processor. Only the current
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processor should be accessing that variable and therefore there are no
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concurrency issues with other processors in the system.
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Please note that accesses by remote processors to a per cpu area are
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exceptional situations and may impact performance and/or correctness
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(remote write operations) of local RMW operations via this_cpu_*.
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The main use of the this_cpu operations has been to optimize counter
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operations.
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The following this_cpu() operations with implied preemption protection
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are defined. These operations can be used without worrying about
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preemption and interrupts::
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this_cpu_read(pcp)
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this_cpu_write(pcp, val)
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this_cpu_add(pcp, val)
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this_cpu_and(pcp, val)
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this_cpu_or(pcp, val)
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this_cpu_add_return(pcp, val)
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this_cpu_xchg(pcp, nval)
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this_cpu_cmpxchg(pcp, oval, nval)
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this_cpu_sub(pcp, val)
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this_cpu_inc(pcp)
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this_cpu_dec(pcp)
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this_cpu_sub_return(pcp, val)
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this_cpu_inc_return(pcp)
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this_cpu_dec_return(pcp)
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Inner working of this_cpu operations
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------------------------------------
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On x86 the fs: or the gs: segment registers contain the base of the
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per cpu area. It is then possible to simply use the segment override
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to relocate a per cpu relative address to the proper per cpu area for
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the processor. So the relocation to the per cpu base is encoded in the
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instruction via a segment register prefix.
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For example::
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DEFINE_PER_CPU(int, x);
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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
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from that address which occurs with the per cpu operations. Before
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this_cpu_ops such sequence also required preempt disable/enable to
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prevent the kernel from moving the thread to a different processor
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while the calculation is performed.
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Consider the following this_cpu operation::
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this_cpu_inc(x)
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The above 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
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register::
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int *y;
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int cpu;
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cpu = get_cpu();
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y = per_cpu_ptr(&x, cpu);
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(*y)++;
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put_cpu();
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Note that these operations can only be used on per cpu data that is
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reserved for a specific processor. Without disabling preemption in the
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surrounding code this_cpu_inc() will only guarantee that one of the
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per cpu counters is correctly incremented. However, there is no
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guarantee that the OS will not move the process directly before or
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after the this_cpu instruction is executed. In general this means that
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the value of the individual counters for each processor are
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meaningless. The sum of all the per cpu counters is the only value
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that is of interest.
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Per cpu variables are used for performance reasons. Bouncing cache
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lines can be avoided if multiple processors concurrently go through
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the same code paths. Since each processor has its own per cpu
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variables no concurrent cache line updates take place. The price that
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has to be paid for this optimization is the need to add up the per cpu
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counters when the value of a counter is needed.
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Special operations
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------------------
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::
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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
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of the per cpu variable that belongs to the currently executing
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processor. this_cpu_ptr avoids multiple steps that the common
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get_cpu/put_cpu sequence requires. No processor number is
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available. Instead, the offset of the local per cpu area is simply
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added to the per cpu offset.
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Note that this operation is usually used in a code segment when
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preemption has been disabled. The pointer is then used to
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access local per cpu data in a critical section. When preemption
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is re-enabled this pointer is usually no longer useful since it may
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no longer point to per cpu data of the current processor.
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Per cpu variables and offsets
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-----------------------------
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Per cpu variables have *offsets* to the beginning of the per cpu
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area. They do not have addresses although they look like that in the
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code. Offsets cannot be directly dereferenced. The offset must be
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added to a base pointer of a per cpu area of a processor in order to
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form a valid address.
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Therefore the use of x or &x outside of the context of per cpu
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operations is invalid and will generally be treated like a NULL
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pointer dereference.
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::
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DEFINE_PER_CPU(int, x);
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In the context of per cpu operations the above implies that x is a per
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cpu variable. Most this_cpu operations take a cpu variable.
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::
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int __percpu *p = &x;
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&x and hence p is the *offset* of a per cpu variable. this_cpu_ptr()
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takes the offset of a per cpu variable which makes this look a bit
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strange.
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Operations on a field of a per cpu structure
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--------------------------------------------
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Let's say we have a percpu structure::
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struct s {
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int n,m;
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};
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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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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()
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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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------------------------
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this_cpu ops are interrupt safe. Some architectures do not support
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these per cpu local operations. In that case the operation must be
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replaced by code that disables interrupts, then does the operations
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that are guaranteed to be atomic and then re-enable interrupts. Doing
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so is expensive. If there are other reasons why the scheduler cannot
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change the processor we are executing on then there is no reason to
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disable interrupts. For that purpose the following __this_cpu operations
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are provided.
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These operations have no guarantee against concurrent interrupts or
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preemption. If a per cpu variable is not used in an interrupt context
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and the scheduler cannot preempt, then they are safe. If any interrupts
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still occur while an operation is in progress and if the interrupt too
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modifies the variable, then RMW actions can not be guaranteed to be
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safe::
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__this_cpu_read(pcp)
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__this_cpu_write(pcp, val)
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__this_cpu_add(pcp, val)
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__this_cpu_and(pcp, val)
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__this_cpu_or(pcp, val)
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__this_cpu_add_return(pcp, val)
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__this_cpu_xchg(pcp, nval)
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__this_cpu_cmpxchg(pcp, oval, nval)
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__this_cpu_sub(pcp, val)
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__this_cpu_inc(pcp)
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__this_cpu_dec(pcp)
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__this_cpu_sub_return(pcp, val)
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__this_cpu_inc_return(pcp)
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__this_cpu_dec_return(pcp)
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Will increment x and will not fall-back to code that disables
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interrupts on platforms that cannot accomplish atomicity through
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address relocation and a Read-Modify-Write operation in the same
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instruction.
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&this_cpu_ptr(pp)->n vs this_cpu_ptr(&pp->n)
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--------------------------------------------
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The first operation takes the offset and forms an address and then
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adds the offset of the n field. This may result in two add
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instructions emitted by the compiler.
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The second one first adds the two offsets and then does the
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relocation. IMHO the second form looks cleaner and has an easier time
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with (). The second form also is consistent with the way
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this_cpu_read() and friends are used.
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Remote access to per cpu data
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------------------------------
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Per cpu data structures are designed to be used by one cpu exclusively.
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If you use the variables as intended, this_cpu_ops() are guaranteed to
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be "atomic" as no other CPU has access to these data structures.
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There are special cases where you might need to access per cpu data
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structures remotely. It is usually safe to do a remote read access
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and that is frequently done to summarize counters. Remote write access
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something which could be problematic because this_cpu ops do not
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have lock semantics. A remote write may interfere with a this_cpu
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RMW operation.
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Remote write accesses to percpu data structures are highly discouraged
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unless absolutely necessary. Please consider using an IPI to wake up
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the remote CPU and perform the update to its per cpu area.
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To access per-cpu data structure remotely, typically the per_cpu_ptr()
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function is used::
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DEFINE_PER_CPU(struct data, datap);
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struct data *p = per_cpu_ptr(&datap, cpu);
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This makes it explicit that we are getting ready to access a percpu
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area remotely.
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You can also do the following to convert the datap offset to an address::
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struct data *p = this_cpu_ptr(&datap);
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but, passing of pointers calculated via this_cpu_ptr to other cpus is
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unusual and should be avoided.
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Remote access are typically only for reading the status of another cpus
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per cpu data. Write accesses can cause unique problems due to the
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relaxed synchronization requirements for this_cpu operations.
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One example that illustrates some concerns with write operations is
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the following scenario that occurs because two per cpu variables
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share a cache-line but the relaxed synchronization is applied to
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only one process updating the cache-line.
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Consider the following example::
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struct test {
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atomic_t a;
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int b;
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};
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DEFINE_PER_CPU(struct test, onecacheline);
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There is some concern about what would happen if the field 'a' is updated
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remotely from one processor and the local processor would use this_cpu ops
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to update field b. Care should be taken that such simultaneous accesses to
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data within the same cache line are avoided. Also costly synchronization
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may be necessary. IPIs are generally recommended in such scenarios instead
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of a remote write to the per cpu area of another processor.
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Even in cases where the remote writes are rare, please bear in
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mind that a remote write will evict the cache line from the processor
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that most likely will access it. If the processor wakes up and finds a
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missing local cache line of a per cpu area, its performance and hence
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the wake up times will be affected.
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