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

[Matt Fleming]

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@xxxxxxxxx>
> > 
> > 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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