Merge branch 'rfc/perf' into perf/core, because it's ready for inclusion

 * Copyright (C) 2011-2012 Peter Zijlstra <pzijlstr@redhat.com>

 *

 * Jump labels provide an interface to generate dynamic branches using

 * self-modifying code. Assuming toolchain and architecture support the result

 * of a "if (static_key_false(&key))" statement is a unconditional branch (which

 * self-modifying code. Assuming toolchain and architecture support, the result

 * of a "if (static_key_false(&key))" statement is an unconditional branch (which

 * defaults to false - and the true block is placed out of line).

 *

 * However at runtime we can change the branch target using

 * static_key_slow_{inc,dec}(). These function as a 'reference' count on the key

 * object and for as long as there are references all branches referring to

 * object, and for as long as there are references all branches referring to

 * that particular key will point to the (out of line) true block.

 *

 * Since this relies on modifying code the static_key_slow_{inc,dec}() functions

 * Since this relies on modifying code, the static_key_slow_{inc,dec}() functions

 * must be considered absolute slow paths (machine wide synchronization etc.).

 * OTOH, since the affected branches are unconditional their runtime overhead

 * OTOH, since the affected branches are unconditional, their runtime overhead

 * will be absolutely minimal, esp. in the default (off) case where the total

 * effect is a single NOP of appropriate size. The on case will patch in a jump

 * to the out-of-line block.

 *

 * When the control is directly exposed to userspace it is prudent to delay the

 * When the control is directly exposed to userspace, it is prudent to delay the

 * decrement to avoid high frequency code modifications which can (and do)

 * cause significant performance degradation. Struct static_key_deferred and

 * static_key_slow_dec_deferred() provide for this.

 *

 * Lacking toolchain and or architecture support, it falls back to a simple

 * Lacking toolchain and or architecture support, jump labels fall back to a simple

 * conditional branch.

 *

 * struct static_key my_key = STATIC_KEY_INIT_TRUE;

 *

 * Not initializing the key (static data is initialized to 0s anyway) is the

 * same as using STATIC_KEY_INIT_FALSE.

 *

 */

#include <linux/types.h>