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Subject[PATCH 0/5] ftrace: to kill a daemon

One of the things that bothered me about the latest ftrace code was
this annoying daemon that would wake up once a second to see if it
had work to do. If it did not, it would go to sleep, otherwise it would
do its work and then go to sleep.

You see, the reason for this is that for ftrace to maintain performance
when configured in but disabled, it would need to change all the
locations that called "mcount" (enabled with the gcc -pg option) into
nops. The "-pg" option in gcc sets up a function profiler to call this
function called "mcount". If you simply have "mcount" return, it will
still add 15 to 18% overhead in performance. Changing all the calls to
nops moved the overhead into noise.

To get rid of this, I had the mcount code record the location that called
it. Later, the "ftraced" daemon would wake up and look to see if
any new functions were recorded. If so, it would call kstop_machine
and convert the calls to "nops". We needed kstop_machine because bad
things happen on SMP if you modify code that happens to be in the
instruction cache of another CPU.

This "ftraced" kernel thread would be a happy little worker, but it caused
some pains. One, is that it woke up once a second, and Ted Tso got mad
at me because it would show up on PowerTop. I could easily make the
default 5 seconds, and even have it runtime configurable, with a trivial
patch. I have not got around to doing that yet.

The other annoying thing, and this one bothers me the most, is that we
can not have this enabled on a production -rt kernel. The latency caused
by the kstop_machine when doing work is lost in the noise on a non-rt
kernel, but it can be up to 800 microseconds, and that would kill
the -rt kernel. The reason this bothered me the most, is that -rt is
where it came from, and ftraced was not treating its motherland very well.

Along came Gregory Haskins, who was bickering about having ftrace enabled
on a production -rt kernel. I told him the reasons that this would be bad
and then he started thinking out loud, and suggesting wild ideas, like
patching gcc!

Since I have recently seen "The Dark Knight", Gregory's comments put me
into an "evil" mood. I then thought of the idea about using the
relocation entries of the mcount call sites, in a prelinked object file,
and create a separate section with a list of these sites. On boot up,
record them and change them into nops.

That's it! No kstop_machine for turning them into nops. We would only need
stop_machine to enable or disable tracing, but a user not tracing will not have
to deal with this annoying "ftraced" kernel thread waking up every second
or ever running kstop_machine.

What's more, this means we can enable it on a production -rt kernel!

Now, this was no easy task. We needed to add a section to every object
file with a list of pointers to the call sites to mcount. The idea I came
up with was to make a tmp.s file for every object just after it is compiled.
This tmp.s would then be compiled and relinked into the original object.
The tmp.s file would have something like:

.section __mcount_loc,"a",@progbits
.quad location_of_mcount1
.quad location_of_mcount2

By running objdump on the object file we can find the offsets into the
sections that the functions are called.

For example, looking at hrtimer.o:

Disassembly of section .text:

0000000000000000 <hrtimer_init_sleeper>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: e8 00 00 00 00 callq 9 <hrtimer_init_sleeper+0x9>
5: R_X86_64_PC32 mcount+0xfffffffffffffffc

the '5' in the '5: R_X86_64_PC32' is the offset that the mcount relocation
is to be done for the call site. This offset is from the .text section,
and not necessarily, from the function. If we look further we see:

000000000000001e <ktime_add_safe>:
1e: 55 push %rbp
1f: 48 89 e5 mov %rsp,%rbp
22: e8 00 00 00 00 callq 27 <ktime_add_safe+0x9>
23: R_X86_64_PC32 mcount+0xfffffffffffffffc

This mcount call site is 0x23 from the .text section, and obviously
not from the ktime_add_safe.

If we make a tmp.s that has the following:

.section __mcount_loc,"a",@progbits
.quad hrtimer_init_sleeper + 0x5
.quad hrtimer_init_sleeper + 0x23

We have a section with the locations of these two call sites. After the final
linking, they will point to the actual address used.

All that would need to be done is:

gcc -c tmp.s -o tmp.o
ld -r tmp.o hrtimer.o -o tmp_hrtime.o
mv tmp_hrtimer.o hrtimer.o

Easy as that! Not quite. What happens if that first function in the
section is a static function? That is, the symbol for the function
is local to the object. If for some reason hrtimer_init_sleeper is static,
the tmp_hrtimer.o would have two symbols for hrtimer_init_sleeper.
One local and one global.

But we can be even more evil with this idea. We can do crazy things
with objcopy to solve it for us.

objcopy --globalize-symbol hrtimer_init_sleeper hrtimer.o tmp_hrtimer.o

Now the hrtimer_init_sleeper would be global for linking.

ld -r tmp_hrtimer.o tmp.o -o tmp2_hrtimer.o

Now the tmp.o could use the same global hrtimer_init_sleeper symbol.
But we have tmp2_hritmer.o that has the tmp.o and tmp_hrtimer.o symbols,
but we cant just blindly convert local symbols to globals.

The solution is simply put it back to local.

objcopy --localize-symbol hrtimer_init_sleeper tmp2_hrtimer.o hrtimer.o

Now our hrtimer.o file has our __mcount_loc section and the
reference to hrtimer_init_sleeper will be resolved.

This is a bit complex to do in shell scripting and Makefiles, so I wrote
a well documented perl script, that will do the above
all in one place.

With this new update, we can work to kill that kernel thread "ftraced"!

This patch set ports to x86_64 and i386, the other archs will still use
the daemon until they are converted over.

I tested this on both x86_64 and i386 with and without CONFIG_RELOCATE

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