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