lkml.org 
[lkml]   [2007]   [Jul]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[patch 08/10] Immediate Value - Documentation
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
---
Documentation/immediate.txt | 103 ++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 103 insertions(+)

Index: linux-2.6-lttng/Documentation/immediate.txt
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6-lttng/Documentation/immediate.txt 2007-06-15 16:14:05.000000000 -0400
@@ -0,0 +1,103 @@
+ Using the Immediate Values
+
+ Mathieu Desnoyers
+
+
+This document introduces Immediate Values and their use.
+
+* Purpose of immediate values
+
+An immediate value is used to compile into the kernel a branch that is disabled
+at compile-time that has almost no measurable performance impact on the kernel.
+Then, at runtime, it can be enabled dynamically.
+
+It can be used to compile code in the kernel that is seldomly meant to be
+dynamically activated. It's the case of CPU specific workarounds, profiling,
+tracing, etc.
+
+This infrastructure is specialized in supporting dynamic patching of the values
+in the instruction stream when multiple CPUs are running without disturbing the
+normal system behavior.
+
+
+* Usage
+
+In order to use the macro immediate, you should include linux/immediate.h.
+
+#include <linux/immediate.h>
+
+immediate_t __read_mostly this_immediate;
+EXPORT_SYMBOL(this_immediate);
+
+
+Add, in your code :
+
+if (unlikely(immediate(this_immediate))) {
+ some code...
+}
+
+And then, use:
+
+Use immediate_arm(&this_immediate) to activate the immediate value.
+
+Use immediate_disarm(&this_immediate) to deactivate the immediate value.
+
+Use immediate_query(&this_immediate) to query the immediate value state.
+
+The immediate mechanism supports inserting multiple instances of the same
+immediate. Immediate values can be put in inline functions, inlined static
+functions, and unrolled loops.
+
+
+* Optimization for a given architecture
+
+One can implement optimized immediate values for a given architecture by
+replacing asm-$ARCH/immediate.h.
+
+The IF_* flags can be used to control the type of immediate value. See the
+include/linux/immediate.h header for the list of flags. They can be specified as
+the first parameter of the _immediate() macro, as in the following example,
+which uses flags to declare a immediate that always uses the generic version of
+the immediates. It can be useful to use this when immediates are placed in
+kernel code presenting particular reentrancy challenges.
+
+if (unlikely(_immediate(IF_DEFAULT & ~IF_OPTIMIZED, this_immediate))) {
+ some code...
+}
+
+
+* Performance improvement
+
+Result of a small test comparing:
+
+1 - Branch depending on a cache miss (has to fetch in memory, caused by a 128
+ bytes stride)). This is the test that is likely to look like what
+ side-effect the original profile_hit code was causing, under the
+ assumption that the kernel is already using L1 and L2 caches at
+ their full capacity and that a supplementary data load would cause
+ cache trashing.
+2 - Branch depending on L1 cache hit. Just for comparison.
+3 - Branch depending on a load immediate in the instruction stream.
+
+It has been compiled with gcc -O2. Tests done on a 3GHz P4.
+
+In the first test series, the branch is not taken:
+
+number of tests : 1000
+number of branches per test : 81920
+memory hit cycles per iteration (mean) : 48.252
+L1 cache hit cycles per iteration (mean) : 16.1693
+instruction stream based test, cycles per iteration (mean) : 16.0432
+
+
+In the second test series, the branch is taken and an integer is
+incremented within the block:
+
+number of tests : 1000
+number of branches per test : 81920
+memory hit cycles per iteration (mean) : 48.2691
+L1 cache hit cycles per iteration (mean) : 16.396
+instruction stream based test, cycles per iteration (mean) : 16.0441
+
+Therefore, the memory fetch based test seems to be 200% slower than the
+load immediate based test.
--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2007-07-03 18:51    [W:0.152 / U:9.664 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site