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

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Message-ID: <20140107083803.GM30183@twins.programming.kicks-ass.net>
Date:	Tue, 7 Jan 2014 09:38:03 +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

On Mon, Jan 06, 2014 at 03:10:28PM -0800, Andi Kleen wrote:
> Peter Zijlstra <peterz@...radead.org> writes:
> > Also, do clarify the other points I asked about. Esp. the non
> > FREEZE_ON_PMI behaviour of the PT PMI is worrying me immensely.
> 
> The only reason for hardware freeze is when you have a few entries (like
> with LBRs) so the interrupt entry code could overwhelm it.
> 
> But PT is not small, it's gigantic: even with the smallest buffer you
> have many thousands of entries.
> 
> So you will get a few branches in the interrupt entry, but it's not a problem
> because everything you really wanted to trace is still there.
> 
> Eventually the handler disables PT, so there's no risk of racing with
> the update or anything like that.
> 
> Did I miss anything?

Yes; go read this:

 lkml.kernel.org/r/20131219125205.GT3694@...ns.programming.kicks-ass.net
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