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authorLinus Torvalds <torvalds@linux-foundation.org>2017-07-15 22:58:58 +0300
committerLinus Torvalds <torvalds@linux-foundation.org>2017-07-15 22:58:58 +0300
commit486088bc4689f826b80aa317b45ac9e42e8b25ee (patch)
treeadf5847a6119d24da990d9e336f005c4a316e6be /Documentation/this_cpu_ops.txt
parent52f6c588c77b76d548201470c2a28263a41b462b (diff)
parent43e5f7e1fa66531777c49791014c3124ea9208d8 (diff)
downloadlinux-486088bc4689f826b80aa317b45ac9e42e8b25ee.tar.xz
Merge tag 'standardize-docs' of git://git.lwn.net/linux
Pull documentation format standardization from Jonathan Corbet: "This series converts a number of top-level documents to the RST format without incorporating them into the Sphinx tree. The hope is to bring some uniformity to kernel documentation and, perhaps more importantly, have our existing docs serve as an example of the desired formatting for those that will be added later. Mauro has gone through and fixed up a lot of top-level documentation files to make them conform to the RST format, but without moving or renaming them in any way. This will help when we incorporate the ones we want to keep into the Sphinx doctree, but the real purpose is to bring a bit of uniformity to our documentation and let the top-level docs serve as examples for those writing new ones" * tag 'standardize-docs' of git://git.lwn.net/linux: (84 commits) docs: kprobes.txt: Fix whitespacing tee.txt: standardize document format cgroup-v2.txt: standardize document format dell_rbu.txt: standardize document format zorro.txt: standardize document format xz.txt: standardize document format xillybus.txt: standardize document format vfio.txt: standardize document format vfio-mediated-device.txt: standardize document format unaligned-memory-access.txt: standardize document format this_cpu_ops.txt: standardize document format svga.txt: standardize document format static-keys.txt: standardize document format smsc_ece1099.txt: standardize document format SM501.txt: standardize document format siphash.txt: standardize document format sgi-ioc4.txt: standardize document format SAK.txt: standardize document format rpmsg.txt: standardize document format robust-futexes.txt: standardize document format ...
Diffstat (limited to 'Documentation/this_cpu_ops.txt')
-rw-r--r--Documentation/this_cpu_ops.txt49
1 files changed, 28 insertions, 21 deletions
diff --git a/Documentation/this_cpu_ops.txt b/Documentation/this_cpu_ops.txt
index 2cbf71975381..5cb8b883ae83 100644
--- a/Documentation/this_cpu_ops.txt
+++ b/Documentation/this_cpu_ops.txt
@@ -1,5 +1,9 @@
+===================
this_cpu operations
--------------------
+===================
+
+:Author: Christoph Lameter, August 4th, 2014
+:Author: Pranith Kumar, Aug 2nd, 2014
this_cpu operations are a way of optimizing access to per cpu
variables associated with the *currently* executing processor. This is
@@ -39,7 +43,7 @@ operations.
The following this_cpu() operations with implied preemption protection
are defined. These operations can be used without worrying about
-preemption and interrupts.
+preemption and interrupts::
this_cpu_read(pcp)
this_cpu_write(pcp, val)
@@ -67,14 +71,14 @@ 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:
+For example::
DEFINE_PER_CPU(int, x);
int z;
z = this_cpu_read(x);
-results in a single instruction
+results in a single instruction::
mov ax, gs:[x]
@@ -84,16 +88,16 @@ 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:
+Consider the following this_cpu operation::
this_cpu_inc(x)
-The above results in the following single instruction (no lock prefix!)
+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:
+register::
int *y;
int cpu;
@@ -121,8 +125,10 @@ 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:
--------------------
+Special operations
+------------------
+
+::
y = this_cpu_ptr(&x)
@@ -153,11 +159,15 @@ 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()
@@ -168,7 +178,7 @@ strange.
Operations on a field of a per cpu structure
--------------------------------------------
-Let's say we have a percpu structure
+Let's say we have a percpu structure::
struct s {
int n,m;
@@ -177,14 +187,14 @@ Let's say we have a percpu structure
DEFINE_PER_CPU(struct s, p);
-Operations on these fields are straightforward
+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:
+If we have an offset to struct s::
struct s __percpu *ps = &p;
@@ -194,7 +204,7 @@ If we have an offset to struct s:
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:
+if we do not make use of this_cpu ops later to manipulate fields::
struct s *pp;
@@ -206,7 +216,7 @@ if we do not make use of this_cpu ops later to manipulate fields:
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
@@ -222,7 +232,7 @@ 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.
+safe::
__this_cpu_read(pcp)
__this_cpu_write(pcp, val)
@@ -279,7 +289,7 @@ unless absolutely necessary. Please consider using an IPI to wake up
the remote CPU and perform the update to its per cpu area.
To access per-cpu data structure remotely, typically the per_cpu_ptr()
-function is used:
+function is used::
DEFINE_PER_CPU(struct data, datap);
@@ -289,7 +299,7 @@ function is used:
This makes it explicit that we are getting ready to access a percpu
area remotely.
-You can also do the following to convert the datap offset to an address
+You can also do the following to convert the datap offset to an address::
struct data *p = this_cpu_ptr(&datap);
@@ -305,7 +315,7 @@ the following scenario that occurs because two per cpu variables
share a cache-line but the relaxed synchronization is applied to
only one process updating the cache-line.
-Consider the following example
+Consider the following example::
struct test {
@@ -327,6 +337,3 @@ mind that a remote write will evict the cache line from the processor
that most likely will access it. If the processor wakes up and finds a
missing local cache line of a per cpu area, its performance and hence
the wake up times will be affected.
-
-Christoph Lameter, August 4th, 2014
-Pranith Kumar, Aug 2nd, 2014