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

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Message-ID: <20090417194613.GA30544@elte.hu>
Date:	Fri, 17 Apr 2009 21:46:13 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
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


* 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.

	Ingo
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