tales.md

Tales from the kernel

Some random tales from the kernel.

April 2018 - static keys

Reference: Documentation/static-keys.txt

Static keys allows the inclusion of seldom used features in performance-sensitive fast-path kernel code, via a GCC feature and a code patching technique. A quick example::

DEFINE_STATIC_KEY_FALSE(key);

...

    if (static_branch_unlikely(&key))
            do unlikely code
    else
            do likely code

...
static_branch_enable(&key);
...
static_branch_disable(&key);
...

The static_branch_unlikely() branch will be generated into the code with as little impact to the likely code path as possible.

Implementation

(paraphrased by Tobin)

static_branch_unlikely() compiles the likely code inline and adds a (5 byte) noop instruction. If static_branch_enable() is called the instructions are overwritten with a jump to out of line code.