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

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Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Ingo Molnar 2014-08-24 22:35:42 +02:00
commit 39b5a56ec0
1 changed files with 8 additions and 9 deletions
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17
include/linux/jump_label.h
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@ -8,28 +8,28 @@
* 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;
@ -43,8 +43,7 @@
*
* Not initializing the key (static data is initialized to 0s anyway) is the
* same as using STATIC_KEY_INIT_FALSE.
*
*/
*/
#include <linux/types.h>
#include <linux/compiler.h>