very_unlikely() very unliked

The facility formerly known as jump labels is designed to minimize the cost of rarely-used features in the kernel. The classic example is tracepoints, which are turned off almost all of the time. Ideally, the overhead of a disabled tracepoint would be zero, or something very close to it. Jump labels (now "static keys") use runtime code modification to patch unused features out of the code path entirely until somebody turns them on.

With Ingo Molnar's encouragement, Jason Baron recently posted a patch set changing the jump label mechanism to look like this:

    if (very_unlikely(&jump_label_key)) {
	/* Rarely-used code appears here */
    }

A number of reviewers complained, saying that very_unlikely() looks like just a stronger form of the unlikely() macro, which works in a different way. Ingo responded that the resemblance was not coincidental and made sense, but he made few converts. After arguing the point for a while, Ingo gave in to the inevitable and stopped pushing for the very_unlikely() name.

Instead, we have yet another format for jump labels, as described in this document. A jump label static key is defined with one of:

    struct static_key key = STATIC_KEY_INIT_FALSE;
    struct static_key key = STATIC_KEY_INIT_TRUE;

The initial value should match the normal operational setting - the one that should be fast. Testing of static keys is done with:

    if (static_key_false(&key)) {
	/* Unlikely code here */
    }

Note that static_key_false() can only be used with a key initialized with STATIC_KEY_INIT_FALSE; for default-true keys, static_key_true() must be used instead. To make a key true, pass it to static_key_slow_inc(); removing a reference to a key (and possibly making it false) is done with static_key_slow_dec().

This version of the change was rather less controversial and, presumably, will find its way in through the 3.4 merge window. After that, one assumes, this mechanism will not be reworked again for at least a development cycle or two.

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