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Message-ID: <20091027065421.GA22032@uhli> Date: Tue, 27 Oct 2009 14:54:21 +0800 From: Li Hong <lihong.hi@...il.com> To: Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org Subject: [PATCH 1/9] tracing: recordmcount.pl Amend the documentation according to the implementation In documentation, we says we will use the first function in a section as a reference. Actually, the algorithm is: choose the first global function we meet as a reference. If there is none, choose the first local one. So let the documentation consistent to the code. Also add several clarifications. Signed-off-by: Li Hong <lihong.hi@...il.com> diff --git a/scripts/recordmcount.pl b/scripts/recordmcount.pl index 090d300..ac908b3 100755 --- a/scripts/recordmcount.pl +++ b/scripts/recordmcount.pl @@ -6,73 +6,88 @@ # 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 a data area in .init.data in vmlinux +# 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. # # 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. # -# 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. +# 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 refering the start of a section to +# refering a function symbol in this section. During the link step, 'ld' will +# compute the final address according to the information we record. +# # 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 +101,8 @@ # 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; -- 1.6.0.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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