[PATCH 12/12] jump label v10: add docs
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From: Jason Baron on 27 Jul 2010 17:00 Add jump label docs as: Documentation/jump-label.txt Signed-off-by: Jason Baron <jbaron(a)redhat.com> --- Documentation/jump-label.txt | 151 ++++++++++++++++++++++++++++++++++++++++++ 1 files changed, 151 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..232ab79 --- /dev/null +++ b/Documentation/jump-label.txt @@ -0,0 +1,151 @@ + Jump Label + ---------- + +By: Jason Baron <jbaron(a)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. In the case of !CC_OPTIMIZE_FOR_SIZE this +optimization is seen on high level benchmarks such as tbench where I can get +~1-2% higher throughput. In addition we are within .5% of the performance of no +tracepoints compiled in at all. + + +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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |