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

Jeremy Fitzhardinge
2009-04-17T19:59:17Z

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.

> also, did you investigate the effect on the _instrumented_ function
> itself? (i.e. the non-tracing related bits) A function call clobbers
> various registers and creates pressure on gcc to shuffle registers
> around.
>

Well, there's a function call in either case, so I don't think it makes
much difference.

J