lkml.org 
[lkml]   [2009]   [Oct]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 1/5] [PATCH 1/5] function-graph/x86: replace unbalanced ret with jmp
* Steven Rostedt (rostedt@goodmis.org) wrote:
> On Tue, 2009-10-13 at 16:47 -0400, Mathieu Desnoyers wrote:
> > * Steven Rostedt (rostedt@goodmis.org) wrote:
> > > From: Steven Rostedt <srostedt@redhat.com>
> > >
> > > The function graph tracer replaces the return address with a hook to
> > > trace the exit of the function call. This hook will finish by returning
> > > to the real location the function should return to.
> > >
> > > But the current implementation uses a ret to jump to the real return
> > > location. This causes a imbalance between calls and ret. That is
> > > the original function does a call, the ret goes to the handler
> > > and then the handler does a ret without a matching call.
> > >
> > > Although the function graph tracer itself still breaks the branch
> > > predictor by replacing the original ret, by using a second ret and
> > > causing an imbalance, it breaks the predictor even more.
> > >
> > > This patch replaces the ret with a jmp to keep the calls and ret
> > > balanced. I tested this on one box and it showed a 1.7% increase in
> > > performance. Another box only showed a small 0.3% increase. But no
> > > box that I tested this on showed a decrease in performance by making this
> > > change.
> >
> > This sounds exactly like what I proposed at LPC. I'm glad it shows
> > actual improvements.
>
> This is what we discussed at LPC. We both were under the assumption that
> a jump would work. The question was how to make that jump without hosing
> registers.
>
> We lucked out that this is the back end of the return sequence. Where we
> can still clobber callie registers. (just not the ones holding the
> return code).
>
> >
> > Just to make sure I understand, the old sequence was:
> >
> > call fct
> > call ftrace_entry
> > ret to fct
> > ret to ftrace_exit
> > ret to caller
> >
> > and you now have:
> >
> > call fct
> > call ftrace_entry
> > ret to fct
> > ret to ftrace_exit
> > jmp to caller
> >
> > Am I correct ?
>
> Almost.
>
> What it was:
>
> call function
> function:
> call mcount
> mcount:
> call ftrace_entry
> ftrace_entry:
> mess up with return code of caller
> ret
> ret
>
> [function code]
>
> ret to ftrace_exit
> ftrace_exit:
> get real return
> ret to original
>
> So for the function we have 3 calls and 4 rets
>
> Now we have:
>
> What it was:
>
> call function
> function:
> call mcount
> mcount:
> call ftrace_entry

Can we manage to change this call

> ftrace_entry:
> mess up with return code of caller
> ret

.. and this ret for 2 jmp instructions too ?

Given that we have no choice but to kill call/ret prediction logic, I
think it might be good to try to use this logic as little as possible
(by favoring jmp jmp over call/ret when the return target is invariant).

That's just an idea, benchmarks could prove me right/wrong.

Mathieu

> ret
>
> [function code]
>
> ret to ftrace_exit
> ftrace_exit:
> get real return
> jmp to original
>
> Now we have 3 calls and 3 rets
>
> Note the first call still does not match the ret, but we don't do two
> rets anymore.
>
> -- Steve
>
>
>

--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


\
 
 \ /
  Last update: 2009-10-13 23:41    [W:0.090 / U:0.028 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site