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    Subject[tip:tracing/core] tracing: Amend documentation in recordmcount.pl to reflect implementation
    Commit-ID:  d49f6aa76d24c60a52530474cb662e8ad9f09471
    Gitweb: http://git.kernel.org/tip/d49f6aa76d24c60a52530474cb662e8ad9f09471
    Author: Li Hong <lihong.hi@gmail.com>
    AuthorDate: Wed, 28 Oct 2009 13:01:38 +0800
    Committer: Steven Rostedt <rostedt@goodmis.org>
    CommitDate: Thu, 29 Oct 2009 15:11:40 -0400

    tracing: Amend documentation in recordmcount.pl to reflect implementation

    The documentation currently says we will use the first function in a section
    as a reference. The actual algorithm is: choose the first global function we
    meet as a reference. If there is none, choose the first local one.
    Change the documentation to be consistent with the code.

    Also add several other clarifications.

    Signed-off-by: Li Hong <lihong.hi@gmail.com>
    LKML-Reference: <20091028050138.GA30758@uhli>
    Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
    ---
    scripts/recordmcount.pl | 84 +++++++++++++++++++++++++++-------------------
    1 files changed, 49 insertions(+), 35 deletions(-)

    diff --git a/scripts/recordmcount.pl b/scripts/recordmcount.pl
    index bfb8b2c..a569be7 100755
    --- a/scripts/recordmcount.pl
    +++ b/scripts/recordmcount.pl
    @@ -6,73 +6,89 @@
    # all the offsets to the calls to mcount.
    #
    #
    -# What we want to end up with is a section in vmlinux called
    -# __mcount_loc that contains a list of pointers to all the
    -# call sites in the kernel that call mcount. Later on boot up, the kernel
    -# will read this list, save the locations and turn them into nops.
    -# When tracing or profiling is later enabled, these locations will then
    -# be converted back to pointers to some function.
    +# What we want to end up with this is that each object file will have a
    +# section called __mcount_loc that will hold the list of pointers to mcount
    +# callers. After final linking, the vmlinux will have within .init.data the
    +# list of all callers to mcount between __start_mcount_loc and __stop_mcount_loc.
    +# Later on boot up, the kernel will read this list, save the locations and turn
    +# them into nops. When tracing or profiling is later enabled, these locations
    +# will then be converted back to pointers to some function.
    #
    # This is no easy feat. This script is called just after the original
    # object is compiled and before it is linked.
    #
    -# The references to the call sites are offsets from the section of text
    -# that the call site is in. Hence, all functions in a section that
    -# has a call site to mcount, will have the offset from the beginning of
    -# the section and not the beginning of the function.
    +# When parse this object file using 'objdump', the references to the call
    +# sites are offsets from the section that the call site is in. Hence, all
    +# functions in a section that has a call site to mcount, will have the
    +# offset from the beginning of the section and not the beginning of the
    +# function.
    +#
    +# But where this section will reside finally in vmlinx is undetermined at
    +# this point. So we can't use this kind of offsets to record the final
    +# address of this call site.
    +#
    +# The trick is to change the call offset referring the start of a section to
    +# referring a function symbol in this section. During the link step, 'ld' will
    +# compute the final address according to the information we record.
    #
    -# The trick is to find a way to record the beginning of the section.
    -# The way we do this is to look at the first function in the section
    -# which will also be the location of that section after final link.
    # e.g.
    #
    # .section ".sched.text", "ax"
    -# .globl my_func
    -# my_func:
    # [...]
    -# call mcount (offset: 0x5)
    +# func1:
    +# [...]
    +# call mcount (offset: 0x10)
    +# [...]
    +# ret
    +# .globl fun2
    +# func2: (offset: 0x20)
    +# [...]
    # [...]
    # ret
    -# other_func:
    +# func3:
    # [...]
    -# call mcount (offset: 0x1b)
    +# call mcount (offset: 0x30)
    # [...]
    #
    # Both relocation offsets for the mcounts in the above example will be
    -# offset from .sched.text. If we make another file called tmp.s with:
    +# offset from .sched.text. If we choose global symbol func2 as a reference and
    +# make another file called tmp.s with the new offsets:
    #
    # .section __mcount_loc
    -# .quad my_func + 0x5
    -# .quad my_func + 0x1b
    +# .quad func2 - 0x10
    +# .quad func2 + 0x10
    #
    -# We can then compile this tmp.s into tmp.o, and link it to the original
    +# We can then compile this tmp.s into tmp.o, and link it back to the original
    # object.
    #
    -# But this gets hard if my_func is not globl (a static function).
    -# In such a case we have:
    +# In our algorithm, we will choose the first global function we meet in this
    +# section as the reference. But this gets hard if there is no global functions
    +# in this section. In such a case we have to select a local one. E.g. func1:
    #
    # .section ".sched.text", "ax"
    -# my_func:
    +# func1:
    # [...]
    -# call mcount (offset: 0x5)
    +# call mcount (offset: 0x10)
    # [...]
    # ret
    -# other_func:
    +# func2:
    # [...]
    -# call mcount (offset: 0x1b)
    +# call mcount (offset: 0x20)
    # [...]
    +# .section "other.section"
    #
    # If we make the tmp.s the same as above, when we link together with
    -# the original object, we will end up with two symbols for my_func:
    +# the original object, we will end up with two symbols for func1:
    # one local, one global. After final compile, we will end up with
    -# an undefined reference to my_func.
    +# an undefined reference to func1 or a wrong reference to another global
    +# func1 in other files.
    #
    # Since local objects can reference local variables, we need to find
    # a way to make tmp.o reference the local objects of the original object
    -# file after it is linked together. To do this, we convert the my_func
    +# file after it is linked together. To do this, we convert func1
    # into a global symbol before linking tmp.o. Then after we link tmp.o
    -# we will only have a single symbol for my_func that is global.
    -# We can convert my_func back into a local symbol and we are done.
    +# we will only have a single symbol for func1 that is global.
    +# We can convert func1 back into a local symbol and we are done.
    #
    # Here are the steps we take:
    #
    @@ -86,10 +102,8 @@
    # 6) Link together this new object with the list object.
    # 7) Convert the local functions back to local symbols and rename
    # the result as the original object.
    -# End.
    # 8) Link the object with the list object.
    # 9) Move the result back to the original object.
    -# End.
    #

    use strict;

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