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Date: Tue, 7 Jan 2014 09:38:03 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Andi Kleen <andi@firstfloor.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Ingo Molnar <mingo@kernel.org>,
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        Stephane Eranian <eranian@google.com>
Subject: Re: [PATCH v0 04/71] itrace: Infrastructure for instruction flow
 tracing units
Message-ID: <20140107083803.GM30183@twins.programming.kicks-ass.net>
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On Mon, Jan 06, 2014 at 03:10:28PM -0800, Andi Kleen wrote:
> Peter Zijlstra <peterz@infradead.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@twins.programming.kicks-ass.net