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    SubjectRe: [PATCH v6 4/7] arm64: Add ftrace support
    On Thu, Mar 13, 2014 at 10:13:47AM +0000, AKASHI Takahiro wrote:
    > This patch implements arm64 specific part to support function tracers,
    > such as function (CONFIG_FUNCTION_TRACER), function_graph
    > (CONFIG_FUNCTION_GRAPH_TRACER) and function profiler
    > (CONFIG_FUNCTION_PROFILER).
    >
    > With 'function' tracer, all the functions in the kernel are traced with
    > timestamps in ${sysfs}/tracing/trace. If function_graph tracer is
    > specified, call graph is generated.
    >
    > The kernel must be compiled with -pg option so that _mcount() is inserted
    > at the beginning of functions. This function is called on every function's
    > entry as long as tracing is enabled.
    > In addition, function_graph tracer also needs to be able to probe function's
    > exit. ftrace_graph_caller() & return_to_handler do this by faking link
    > register's value to intercept function's return path.
    >
    > More details on architecture specific requirements are described in
    > Documentation/trace/ftrace-design.txt.

    [...]

    > diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S
    > new file mode 100644
    > index 0000000..0ac31c8
    > --- /dev/null
    > +++ b/arch/arm64/kernel/entry-ftrace.S
    > @@ -0,0 +1,175 @@
    > +/*
    > + * arch/arm64/kernel/entry-ftrace.S
    > + *
    > + * Copyright (C) 2013 Linaro Limited
    > + * Author: AKASHI Takahiro <takahiro.akashi@linaro.org>
    > + *
    > + * This program is free software; you can redistribute it and/or modify
    > + * it under the terms of the GNU General Public License version 2 as
    > + * published by the Free Software Foundation.
    > + */
    > +
    > +#include <linux/linkage.h>
    > +#include <asm/ftrace.h>
    > +#include <asm/insn.h>
    > +
    > +/*
    > + * Gcc with -pg will put the following code in the beginning of each function:
    > + * mov x0, x30
    > + * bl _mcount
    > + * [function's body ...]
    > + * "bl _mcount" may be replaced to "bl ftrace_caller" or NOP if dynamic
    > + * ftrace is enabled.
    > + *
    > + * Please note that x0 as an argument will not be used here because we can
    > + * get lr(x30) of instrumented function at any time by winding up call stack
    > + * as long as the kernel is compiled without -fomit-frame-pointer.
    > + * (or CONFIG_FRAME_POINTER, this is forced on arm64)
    > + *
    > + * stack layout after mcount_enter in _mcount():
    > + *
    > + * current sp/fp => 0:+-----+
    > + * in _mcount() | x29 | -> instrumented function's fp
    > + * +-----+
    > + * | x30 | -> _mcount()'s lr (= instrumented function's pc)
    > + * old sp => +16:+-----+
    > + * when instrumented | |
    > + * function calls | ... |
    > + * _mcount() | |
    > + * | |
    > + * instrumented => +xx:+-----+
    > + * function's fp | x29 | -> parent's fp
    > + * +-----+
    > + * | x30 | -> instrumented function's lr (= parent's pc)
    > + * +-----+
    > + * | ... |
    I guess it's just the diff that's misaligning your ASCII art here?

    > +/*
    > + * void return_to_handler(void)
    > + *
    > + * Run ftrace_return_to_handler() before going back to parent.
    > + * @fp is checked against the value passed by ftrace_graph_caller()
    > + * only when CONFIG_FUNCTION_GRAPH_FP_TEST is enabled.
    > + */
    > + .global return_to_handler
    > +return_to_handler:

    ENTRY(return_to_handler)

    > + str x0, [sp, #-16]!
    > + mov x0, x29 // parent's fp
    > + bl ftrace_return_to_handler// addr = ftrace_return_to_hander(fp);
    > + mov x30, x0 // restore the original return address
    > + ldr x0, [sp], #16
    > + ret

    and an ENDPROC here.

    > +#endif /* CONFIG_FUNCTION_GRAPH_TRACER */
    > diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
    > new file mode 100644
    > index 0000000..a559ab8
    > --- /dev/null
    > +++ b/arch/arm64/kernel/ftrace.c
    > @@ -0,0 +1,64 @@
    > +/*
    > + * arch/arm64/kernel/ftrace.c
    > + *
    > + * Copyright (C) 2013 Linaro Limited
    > + * Author: AKASHI Takahiro <takahiro.akashi@linaro.org>
    > + *
    > + * This program is free software; you can redistribute it and/or modify
    > + * it under the terms of the GNU General Public License version 2 as
    > + * published by the Free Software Foundation.
    > + */
    > +
    > +#include <linux/ftrace.h>
    > +#include <linux/swab.h>
    > +#include <linux/uaccess.h>
    > +
    > +#include <asm/cacheflush.h>
    > +#include <asm/ftrace.h>
    > +#include <asm/insn.h>
    > +
    > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
    > +/*
    > + * function_graph tracer expects ftrace_return_to_handler() to be called
    > + * on the way back to parent. For this purpose, this function is called
    > + * in _mcount() or ftrace_caller() to replace return address (*parent) on
    > + * the call stack to return_to_handler.
    > + *
    > + * Note that @frame_pointer is used only for sanity check later.
    > + */
    > +void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr,
    > + unsigned long frame_pointer)
    > +{
    > + unsigned long return_hooker = (unsigned long)&return_to_handler;
    > + unsigned long old;
    > + struct ftrace_graph_ent trace;
    > + int err;
    > +
    > + if (unlikely(atomic_read(&current->tracing_graph_pause)))
    > + return;
    > +
    > + /*
    > + * Note:
    > + * No protection against faulting at *parent, which may be seen
    > + * on other archs. It's unlikely on AArch64.
    > + */
    > + old = *parent;
    > + *parent = return_hooker;

    return_hook? People might take it personally otherwise ;)

    > + trace.func = self_addr;

    in kernel/ftrace/ftrace.c there's an smb_wmb() between setting up
    tracing_graph_pause and setting the ret_stack with a comment saying:

    /* Make sure the tasks see the -1 first: */
    Why don't we have a corresponding read-barrier here?

    Will


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