From a1b2a555d6375f1ed34d0a1761b540de3a8727c6 Mon Sep 17 00:00:00 2001 From: Christoph Lameter Date: Thu, 4 Apr 2013 14:41:08 +0000 Subject: percpu: add documentation on this_cpu operations 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 Signed-off-by: Tejun Heo --- Documentation/this_cpu_ops.txt | 205 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 205 insertions(+) create mode 100644 Documentation/this_cpu_ops.txt (limited to 'Documentation/this_cpu_ops.txt') diff --git a/Documentation/this_cpu_ops.txt b/Documentation/this_cpu_ops.txt new file mode 100644 index 000000000000..1a4ce7e3e05f --- /dev/null +++ b/Documentation/this_cpu_ops.txt @@ -0,0 +1,205 @@ +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 -- cgit v1.2.3-59-g8ed1b