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

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Date:	Fri, 9 Jan 2015 15:24:42 +0000
From:	Matt Fleming <matt@...sole-pimps.org>
To:	Peter Zijlstra <peterz@...radead.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, 09 Jan, at 02:02:50PM, Peter Zijlstra wrote:
> 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?

Right, but now we need a "steal limit", so we know when to stop stealing
active RMIDs.

	(cqm_max_rmid + 1) / 4 ?

I guess any limit is better than no limit.

-- 
Matt Fleming, Intel Open Source Technology Center
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