linux-kernel - Re: [PATCH 09/11] perf/x86/intel: Implement LRU monitoring ID allocation for CQM

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Message-ID: <20141008105301.GQ14343@console-pimps.org>
Date:	Wed, 8 Oct 2014 11:53:01 +0100
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>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Matt Fleming <matt.fleming@...el.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH 09/11] perf/x86/intel: Implement LRU monitoring ID
 allocation for CQM

On Wed, 08 Oct, at 11:51:09AM, Peter Zijlstra wrote:
> On Wed, Sep 24, 2014 at 03:04:13PM +0100, Matt Fleming wrote:
> > From: Matt Fleming <matt.fleming@...el.com>
> > 
> > It's possible to run into issues with re-using unused monitoring IDs
> > because there may be stale cachelines associated with that ID from a
> > previous allocation. This can cause the LLC occupancy values to be
> > inaccurate.
> > 
> > To attempt to mitigate this problem we place the IDs on a least recently
> > used list, essentially a FIFO. The basic idea is that the longer the
> > time period between ID re-use the lower the probability that stale
> > cachelines exist in the cache.
> 
> Do we want to provide a user configurable minumum guaranteed queue time?

Potentially, yeah. That might be better suited as part of the final
patch that includes the rotation code, which already has a delayed
workqueue.

We could add a minimum queue time before we start querying whether the
data occupancy value for an RMID has dropped to zero on all sockets.

That'd save us from an expensive smp_call_function_many() when we're
unlikely to succeed anyway.

-- 
Matt Fleming, Intel Open Source Technology Center
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