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    Subject[PATCH 10/10] jump label v11: add docs
    Add jump label docs as: Documentation/jump-label.txt

    Signed-off-by: Jason Baron <jbaron@redhat.com>
    ---
    Documentation/jump-label.txt | 148 ++++++++++++++++++++++++++++++++++++++++++
    1 files changed, 148 insertions(+), 0 deletions(-)
    create mode 100644 Documentation/jump-label.txt

    diff --git a/Documentation/jump-label.txt b/Documentation/jump-label.txt
    new file mode 100644
    index 0000000..2e5cff6
    --- /dev/null
    +++ b/Documentation/jump-label.txt
    @@ -0,0 +1,148 @@
    + Jump Label
    + ----------
    +
    +By: Jason Baron <jbaron@redhat.com>
    +
    +
    +1) motivation
    +
    +
    +Currently, tracepoints are implemented using a conditional. The conditional
    +check requires checking a global variable for each tracepoint. Although,
    +the overhead of this check is small, it increases under memory pressure. As we
    +increase the number of tracepoints in the kernel this may become more of an
    +issue. In addition, tracepoints are often dormant (disabled), and provide no
    +direct kernel functionality. Thus, it is highly desirable to reduce their
    +impact as much as possible. Although tracepoints are the original motivation
    +for this work, other kernel code paths should be able to make use of the jump
    +label optimization.
    +
    +
    +2) jump label description/usage
    +
    +
    +gcc (v4.5) adds a new 'asm goto' statement that allows branching to a label.
    +http://gcc.gnu.org/ml/gcc-patches/2009-07/msg01556.html
    +
    +Thus, this patch set introduces an architecture specific 'JUMP_LABEL()' macro as
    +follows (x86):
    +
    +# define JUMP_LABEL_INITIAL_NOP ".byte 0xe9 \n\t .long 0\n\t"
    +
    +# define JUMP_LABEL(key, label) \
    + do { \
    + asm goto("1:" \
    + JUMP_LABEL_INITIAL_NOP \
    + ".pushsection __jump_table, \"a\" \n\t"\
    + _ASM_PTR "1b, %l[" #label "], %c0 \n\t" \
    + ".popsection \n\t" \
    + : : "i" (key) : : label); \
    + } while (0)
    +
    +
    +For architectures that have not yet introduced jump label support its simply:
    +
    +#define JUMP_LABEL(key, label) \
    + if (unlikely(*key)) \
    + goto label;
    +
    +which then can be used as:
    +
    + ....
    + JUMP_LABEL(trace_name, trace_label, jump_enabled);
    + printk("not doing tracing\n");
    + return;
    +trace_label:
    + printk("doing tracing: %d\n", file);
    + ....
    +
    +The 'key' argument is thus a pointer to a conditional argument that can be used
    +if the optimization is not enabled. Otherwise, this address serves as a unique
    +key to identify the particular instance of the jump label.
    +
    +Thus, when tracing is disabled, we simply have a no-op followed by a jump around
    +the dormant (disabled) tracing code. The 'JUMP_LABEL()' macro, produces a
    +'jump_table' which has the following format:
    +
    +[instruction address] [jump target] [tracepoint key]
    +
    +Thus, to enable a tracepoint, we simply patch the 'instruction address' with
    +a jump to the 'jump target'.
    +
    +The call to enable a jump label is: enable_jump_label(key); to disable:
    +disable_jump_label(key);
    +
    +
    +3) architecture interface
    +
    +
    +There are a few functions and macros which arches must implement in order to
    +take advantage of this optimization. As previously mentioned, if there is no
    +architecture support we simply fall back to a traditional, load, test, and
    +jump sequence.
    +
    +* add "HAVE_ARCH_JUMP_LABEL" to arch/<arch>/Kconfig to indicate support
    +
    +* #define JUMP_LABEL_NOP_SIZE, arch/x86/include/asm/jump_label.h
    +
    +* #define "JUMP_LABEL(key, label)", arch/x86/include/asm/jump_label.h
    +
    +* void arch_jump_label_transform(struct jump_entry *entry, enum jump_label_type type)
    + see: arch/x86/kernel/jump_label.c
    +
    +* void arch_jump_label_text_poke_early(jump_label_t addr)
    + see: arch/x86/kernel/jump_label.c
    +
    +* finally add a definition for "struct jump_entry".
    + see: arch/x86/include/asm/jump_label.h
    +
    +
    +4) Jump label analysis (x86)
    +
    +
    +I've tested the performance of using 'get_cycles()' calls around the
    +tracepoint call sites. For an Intel Core 2 Quad cpu (in cycles, averages):
    +
    + idle after tbench run
    + ---- ----------------
    +old code 32 88
    +new code 2 4
    +
    +
    +The performance improvement can be reproduced reliably on both Intel and AMD
    +hardware.
    +
    +In terms of code analysis the current code for the disabled case is a 'cmpl'
    +followed by a 'je' around the tracepoint code. so:
    +
    +cmpl - 83 3d 0e 77 87 00 00 - 7 bytes
    +je - 74 3e - 2 bytes
    +
    +total of 9 instruction bytes.
    +
    +The new code is a 'nopl' followed by a 'jmp'. Thus:
    +
    +nopl - 0f 1f 44 00 00 - 5 bytes
    +jmp - eb 3e - 2 bytes
    +
    +total of 7 instruction bytes.
    +
    +So, the new code also accounts for 2 less bytes in the instruction cache per tracepoint.
    +
    +The optimization depends on !CC_OPTIMIZE_FOR_SIZE. When CC_OPTIMIZE_FOR_SIZE is
    +set, gcc does not always out of line the not taken label path in the same way
    +that the "if unlikely()" paths are made out of line. Thus, with
    +CC_OPTIMIZE_FOR_SIZE set, this optimization is not always optimal. This may be
    +solved in subsequent gcc versions, that allow us to move labels out of line,
    +while still optimizing for size.
    +
    +
    +5) Acknowledgments
    +
    +
    +Thanks to Roland McGrath and Richard Henderson for helping come up with the
    +initial 'asm goto' and jump label design.
    +
    +Thanks to Mathieu Desnoyers and H. Peter Anvin for calling attention to this
    +issue, and outlining the requirements of a solution. Mathieu also implemened a
    +solution in the form of the "Immediate Values" work.
    --
    1.7.1


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