linux-kernel - Re: [PATCH] tracing: remove superfluous sub instructions

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Date:	Tue, 18 Jan 2011 17:41:48 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>, mingo@...e.hu,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tracing: remove superfluous sub instructions

On Tue, Jan 18, 2011 at 11:22:42AM -0500, Steven Rostedt wrote:
> On Tue, 2011-01-18 at 16:52 +0100, Frederic Weisbecker wrote:
> > On Tue, Jan 18, 2011 at 04:28:18PM +0100, Jiri Olsa wrote:
> > > hi,
> > > 
> > > I think there's no need for substracting MCOUNT_INSN_SIZE from the
> > > IP parameter before calling the function trace (/graph) handler.
> > > 
> > > Maybe I overlooked something, but all the IP usage I saw ended
> > > up in the kallsyms_lookup function, which does the lookup using the
> > > functions' start/end boundaries to find the correct symbol for pointer.
> > > 
> > > Thus it seems to me there's no point in substracting the
> > > MCOUNT_INSN_SIZE value from the IP parameter.
> > > 
> > > I tested for x86_64 and got proper results, I believe it's
> > > the same case for x86_32.
> > > 
> > > wbr,
> > > jirka
> > > 
> > > 
> > > Signed-off-by: Jiri Olsa <jolsa@...hat.com>
> > 
> > Well, that sounds right after all. If we are only interested in the
> > symbol, the instruction that follows "call mcount" is still relevant
> > as it must belong to the same function.
> > 
> > Steve?
> 
> NAK, it will break function triggers/probes. The "func:traceon" and
> "func:traceoff". They compare the ip to the call location of mcount.
> 
> -- Steve
> 
> 

ops, missed this one..

would it make sense to update the IP inside the function_trace_probe_call
function to save one instruction in the entry code used by all? or it's
not worth it..

jirka
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