Re: [PATCH v0 04/71] itrace: Infrastructure for instruction flowtracing units

[Peter Zijlstra]

On Mon, Jan 06, 2014 at 03:10:28PM -0800, Andi Kleen wrote:
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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