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    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
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