linux-kernel - Re: [PATCH 1/4] tracing: move __DO

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Message-ID: <alpine.DEB.2.00.0904171557060.1016@gandalf.stny.rr.com>
Date:	Fri, 17 Apr 2009 15:57:55 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH 1/4] tracing: move __DO_TRACE out of line


On Fri, 17 Apr 2009, Ingo Molnar wrote:

> 
> * Jeremy Fitzhardinge <jeremy@...p.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?
> 
> 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.

I doubt it will make much difference. The inline version stil has the
function call to the trace point handler.

-- Steve

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