linux-kernel - Re: [PATCH 0/4] x86: Add Cache QoS Monitoring (CQM) support

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Date:	Mon, 6 Jan 2014 21:48:29 +0000
From:	"Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Tejun Heo <tj@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	"Ingo Molnar" <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
	Li Zefan <lizefan@...wei.com>,
	"containers@...ts.linux-foundation.org" 
	<containers@...ts.linux-foundation.org>,
	"cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/4] x86: Add Cache QoS Monitoring (CQM) support

On Mon, 2014-01-06 at 22:26 +0100, Peter Zijlstra wrote:
> On Mon, Jan 06, 2014 at 08:10:45PM +0000, Waskiewicz Jr, Peter P wrote:
> > There is one per logical CPU.  However, in the current generation, they
> > report on the usage of the same L3 cache.  But the CPU takes care of the
> > resolution of which MSR write and read comes from the logical CPU, so
> > software doesn't need to lock access to it from different CPUs.
> 
> What are the rules of RMIDs, I can't seem to find that in the SDM and I
> think you're tagging cachelines with them. Which would mean that in
> order to (re) use them you need a complete cache (L3) wipe.

The cacheline is tagged internally with the RMID as part of the waymask
for the thread in the core.

> Without a wipe you keep having stale entries of the former user and no
> clear indication on when your numbers are any good.

That can happen, yes.  If you have leftover cache data from a process
that died that hasn't been evicted yet and it's assigned to the RMID
you're using, you will see its included cache occupancy to the overall
numbers.

> Also, is there any sane way of shooting down the entire L3?

That is a question I'd punt to hpa, but I'll ask him.  Looking around
though, a WBINVD would certainly nuke things, but would hurt
performance.  We could get creative with INVPCID as a process dies.  Let
me ask him though and see if there's a good way to tidy up.

-PJ

-- 
PJ Waskiewicz				Open Source Technology Center
peter.p.waskiewicz.jr@...el.com		Intel Corp.