linux-kernel - Re: [PATCH v0 04/71] itrace: Infrastructure for instruction flow tracing units

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Message-ID: <20140108083152.GI2480@laptop.programming.kicks-ass.net>
Date:	Wed, 8 Jan 2014 09:31:52 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>, Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v0 04/71] itrace: Infrastructure for instruction flow
 tracing units

restoring the list.. I really should drop all emails you send off list
into /dev/null.

On Wed, Jan 08, 2014 at 09:28:40AM +0100, Peter Zijlstra wrote:
> On Tue, Jan 07, 2014 at 10:23:22PM +0100, Andi Kleen wrote:
> > > Yes we very much rely on the FREEZE bits for LBR. PT and LBR being
> > > mutually exclusive wasn't documented (or I missed it) and completely
> > > blows.
> > 
> > Can you describe why it is a problem? I had considered it only a minor
> > inconvenience, for many things you would use LBRs for PT is far better.
> 
> Because is someone writes a GCC tool using perf-LBR support for some
> basic block analysis, and someone else writes another tool for PT, then
> the first tool magically stops working when the PT tool is started.
> 
> We cannot refuse to create perf-LBR events, because at that time there
> might not be a PT user -- and even if there was one, it might go away.
> 
> But as long as there's a PT user around, the LBR events will not be able
> to be scheduled and will simply starve, for no apparent reason.
> 
> Complete and utterly miserable position.
> 
> And it makes sense to write LBR tools because they cover a much greater
> spread of hardware.
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