linux-kernel - Re: [PATCH 1/4] x86, perf: Use a new PMU ack sequence on Skylake

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Date:	Fri, 16 Oct 2015 18:14:27 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andi Kleen <ak@...ux.intel.com>, Andi Kleen <andi@...stfloor.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/4] x86, perf: Use a new PMU ack sequence on Skylake

On Fri, 2015-10-16 at 17:00 +0200, Peter Zijlstra wrote:
> On Fri, Oct 16, 2015 at 06:35:14AM -0700, Andi Kleen wrote:
> > > > In principle the sequence should work on other CPUs too, but
> > > > since I only tested on Skylake it is only enabled there.
> > > 
> > > I would very much like a reduction of the ack states. You introduced the
> > > late thing, which should also work for everyone, and now you introduce
> > > yet another variant.
> > 
> > Ingo suggested to do it this way. Originally I thought it wasn't needed,
> > but I think now that late-ack made some of the races that eventually
> > caused Skylake LBR to fall over worse. So in hindsight it was a good idea
> > to not use it everywhere. 
> > 
> > > I would very much prefer a single ack scheme if at all possible.
> > 
> > Could enable it everywhere, but then users would need to test it
> > on most types of CPUs, as I can't.
> 
> I think Mike still has a Core2 machine (and I might be able to dig out a
> laptop), Ingo should have a NHM(-EP), I have SNB, IVB-EP, HSW. So if you
> could test at least BDW and SKL we might have decent test coverage.

Yeah, beloved ole Q6600 box, a U4100 lappy too.

	-Mike

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