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SubjectRe: [PATCH 10/10] jump label v11: add docs
On Fri, Sep 17, 2010 at 11:09:29AM -0400, Jason Baron wrote:
> 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);

No. That is three arguments. The macro only has two.

> + 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

Uh, not e9?
> +
> +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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