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

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Date:	Fri, 17 Apr 2009 12:58:48 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Steven Rostedt <rostedt@...dmis.org>,
	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

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?
>   

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
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