Re: [RFC][ PATCH -tip v2 0/7] kprobes: Kprobes jump optimization support

Masami Hiramatsu
2009-06-23T13:49:55Z

Hi Andi,

Andi Kleen wrote:
> Masami Hiramatsu <mhiramat@redhat.com> writes:
>> The gcc's crossjumping unifies equivalent code by inserting indirect
>> jumps which jump into other function body. It is hard to know to where
>> these jumps jump, so I decided to disable it when setting
>> CONFIG_OPTPROBES=y.
>
> That sounds quite bad. Tail call optimization is an important optimization
> that especially on kernel style code (lots of indirect pointers
> and sometimes deep call chains) is very useful. It would be quite
> sad if production kernels would lose that optimization.

I think the crossjumping is not the tail call optimization,
http://gcc.gnu.org/onlinedocs/gcc-3.4.6/gccint/Passes.html

>
> Also tail calls in C should always jump directly to another function,
> so they shouldn't be particularly complex to manage.

Tail call jumps directly into the head of another function,
not the middle. Thus it is safe.

>> I also decided not to optimize probes when it is in functions which
>> will cause exceptions, because the exception in the kernel will jump
>> to a fixup code and the fixup code jumps back to the middle of the
>> same function body.
>
> Note that not only exceptions do that, there are a few other cases
> where jumps in and out of out of line sections happen. You might
> need a more general mechanism to detect this.

As far as I can see (under arch/x86), Almost all fixup entries are
defined with ex_table entries, and others jump to the head of
symbols(or functions). The jumps which jump into the middle of
some functions are what I need to find, and, as far as I know,
those fixup jumps are used with exception tables. Of course,
I might miss some fixup codes, in that case, please let me know:-)

Thank you,