linux-kernel - Re: [PATCH] x86, sched: allow topolgies where NUMA nodes share an LLC

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Date:   Thu, 8 Jun 2017 22:20:41 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "rientjes@...gle.com" <rientjes@...gle.com>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "prarit@...hat.com" <prarit@...hat.com>,
        "toshi.kani@...com" <toshi.kani@...com>,
        "brice.goglin@...il.com" <brice.goglin@...il.com>,
        "hpa@...ux.intel.com" <hpa@...ux.intel.com>,
        "mingo@...nel.org" <mingo@...nel.org>
Subject: Re: [PATCH] x86, sched: allow topolgies where NUMA nodes share an LLC

On Thu, Jun 08, 2017 at 08:08:31PM +0000, Luck, Tony wrote:
> > What does? That does sound broken. How can a cache domain sanely span
> > memory controllers?
> 
> Think "cluster on die" with cores on the socket split into two clusters, but still sharing LLC.

The thing is, cluster-on-die works with the current code, and therefore
seems to modify the SRAT an CPUID information in a consistent manner.

Which in turn seems to suggest the LLC really is split for
cluster-on-die.

This is something new, and the Changelog is absolute crap for not
explaining _anything_.

So while SRAT seems to invent new nodes, the CPUID topology bits still
describes the full LLC, now shared across nodes.

Is this accurate?, do these nodes, as described by SRAT, actually have a
memory controller each? And is the LLC still fully integrated across the
nodes? If so, we need to go fix the scheduler domain topology to put a
cache domain across nodes (which is going to be painful).

Just making the warning go away and not explaining things sucks.