this_cpu_ops.rar_No There There

this_cpu operations ------------------- 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)