linux-kernel - Re: [PATCH v3 10/11] perf/x86/intel: Perform rotation on Intel CQM RMIDs

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Message-ID: <20141107122052.GD3337@twins.programming.kicks-ass.net>
Date:	Fri, 7 Nov 2014 13:20:52 +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 v3 10/11] perf/x86/intel: Perform rotation on Intel CQM
 RMIDs

On Thu, Nov 06, 2014 at 12:23:21PM +0000, Matt Fleming wrote:
> +/*
> + * If we fail to assign a new RMID for intel_cqm_rotation_rmid because
> + * cachelines are still tagged with RMIDs in limbo, we progressively
> + * increment the threshold until we find an RMID in limbo with <=
> + * __intel_cqm_threshold lines tagged. This is designed to mitigate the
> + * problem where cachelines tagged with an RMID are not steadily being
> + * evicted.
> + *
> + * On successful rotations we decrease the threshold back towards zero.
> + *
> + * __intel_cqm_max_threshold provides an upper bound on the threshold,
> + * and is measured in bytes because it's exposed to userland.
> + */
> +static unsigned int __intel_cqm_threshold;
> +static unsigned int __intel_cqm_max_threshold = -1;

Should we initialize that to a finite value? Surely results are absolute
crap if we do indeed reach that max?
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