linux-kernel - Re: [PATCH v4 10/11] perf/x86/intel: Perform rotation on Intel CQM RMIDs

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Message-ID: <20150109130250.GH29390@twins.programming.kicks-ass.net>
Date:	Fri, 9 Jan 2015 14:02:50 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Matt Fleming <matt@...sole-pimps.org>
Cc:	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Kanaka Juvva <kanaka.d.juvva@...el.com>,
	Matt Fleming <matt.fleming@...el.com>
Subject: Re: [PATCH v4 10/11] perf/x86/intel: Perform rotation on Intel CQM
 RMIDs

On Fri, Jan 09, 2015 at 12:14:01PM +0000, Matt Fleming wrote:
> On Tue, 06 Jan, at 06:17:12PM, Peter Zijlstra wrote:
> > 
> > afaict the again label will try and steal yet another rmid, if rmids
> > don't decay fast enough, we could end up with all rmids on the limbo
> > list and none active. Or am I missing something here?
> 
> You're not missing anything, that's true, we will try and steal more
> RMIDs. We could perhaps put a limit on how many RMIDs we're willing to
> steal, but I think it should definitely be > 1 because RMIDs can
> stabilize out of order.
> 
> It's worth pointing out that we only steal more RMIDs if the ones on the
> limbo list have been queued for the "minimum queue time" - it really is
> a last resort.

Do we really care? Why not just hold up everything until the one(s) we
have are low enough?

Yes it all blows, but would not some active be better than none active,
just because the stupid lines aren't clearing fast enough?
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