Re: [PATCH 1/4] tracing: move __DO_TRACE out of line

Steven Rostedt
2009-04-17T20:06:26Z

On Fri, 17 Apr 2009, Jeremy Fitzhardinge wrote:

> Ingo Molnar wrote:
> > * Jeremy Fitzhardinge <jeremy@goop.org> wrote:
> >
> >
> > > Taking __do_trace_sched_switch out of lines inserts this into the hot path
> > > (6 instructions, 31 bytes):
> > >
> > > cmpl $0, __tracepoint_sched_switch+8(%rip) #,
> > > __tracepoint_sched_switch.state
> > > je .L1748 #,
> > > movq -136(%rbp), %rdx # next,
> > > movq -144(%rbp), %rsi # prev,
> > > movq %rbx, %rdi # rq,
> > > call __do_trace_sched_switch #
> > > .L1748:
> > >
> >
> > Hm, why isnt this off-line in the function? It's marked unlikely(), isnt it?
> >
>
> Yes, its unlikely(). I don't know why it doesn't move it; I've never seen
> unlikely() do anything useful.
>

I wounder if it is because it is in context_switch(). Perhaps it moved it
to the end of that function, but being that context_switch() is only used
in schedule, it could have inlined it. The gcc was not smart enough to
move this unlikely down to the end of the schedule function?

-- Steve