linux-kernel - Re: [PATCH V5 11/16] perf, core: Pass perf_sample_data to perf

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Message-ID: <20141007152412.GC5850@twins.programming.kicks-ass.net>
Date:	Tue, 7 Oct 2014 17:24:12 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Liang, Kan" <kan.liang@...el.com>
Cc:	"eranian@...gle.com" <eranian@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"acme@...nel.org" <acme@...nel.org>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	"Yan, Zheng" <zheng.z.yan@...el.com>
Subject: Re: [PATCH V5 11/16] perf, core: Pass perf_sample_data to
 perf_callchain()

I think you're going to have to stop using outlook or whatnot, this is
horrible.

On Tue, Oct 07, 2014 at 03:00:00AM +0000, Liang, Kan wrote:
> > -----Original Message-----
> > From: Peter Zijlstra [mailto:peterz@...radead.org]

> > So I don't like this. Why not use the regular PERF_SAMPLE_BRANCH_STACK
> > output to generate the stuff from? We already have two different means,
> > with different transport, for callchains anyhow, so a third really won't matter.
> 
> I'm not sure what you mean by using the regular
> PERF_SAMPLE_BRANCH_STACK output to generate the stuff from.  But we
> don't need to modify various architectures' perf_callchain_user, if
> that's your concern.  An alternative way is to generate the callchain
> output in a higher level, like perf_callchain.  If there is no frame
> pointer, the entry->nr will be set to MAX+1. So  the perf_callchain
> knows that we need to try LBR callstack if possible.  In
> perf_callchain, it resets entry->nr to old value, and call
> perf_callchain_lbr_callstack to check and fill the callchain struct if
> possible.  The patch is as below.

Please instruct your MUA to wrap at 78 chars.

What I meant was: why can't we use the regular PERF_SAMPLE_BRANCH_STACK
output to generate user traces from?

PERF_SAMPLE_BRANCH_STACK is the 'normal' LBR output format. Clobbering
the callstack output is bad.

> What do you think?

I think it still sucks.. you're still clobbering potentially more useful
data.
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