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Subject: Re: [PATCH v3] x86,sched: allow topologies where NUMA nodes share an
 LLC
To: Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>
References: <20180329000024.GA16648@alison-desk.jf.intel.com>
 <alpine.DEB.2.21.1803291505590.1477@nanos.tec.linutronix.de>
 <20180329134723.GA4043@hirez.programming.kicks-ass.net>
Cc: Alison Schofield <alison.schofield@intel.com>,
        Ingo Molnar <mingo@kernel.org>, Tony Luck <tony.luck@intel.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        "H. Peter Anvin" <hpa@linux.intel.com>, Borislav Petkov <bp@alien8.de>,
        David Rientjes <rientjes@google.com>,
        Igor Mammedov <imammedo@redhat.com>,
        Prarit Bhargava <prarit@redhat.com>, brice.goglin@gmail.com,
        x86@kernel.org, linux-kernel@vger.kernel.org
From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <596dcb0c-5bbc-f3be-203b-571170ee8f11@linux.intel.com>
Date: Thu, 29 Mar 2018 07:37:29 -0700
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On 03/29/2018 06:47 AM, Peter Zijlstra wrote:
> Further I think Dave argued that we should not change the llc-size,
> because while SNC presents a subset of the cache to local CPUs, for
> remote data the whole cache is still available, again something some
> applications might rely on.

BTW, I may have argued this in the past, but I don't think it's the best
thing to do.

If anything, we should optimize for the _users_ of this information: the
performance-sensitive ones who are digging up the cache topology.  They
are also likely to be the most NUMA-affinitized and stay node-local with
much of their memory traffic.  That would seem to point us in the
direction of enumerating two separate, half-sized LLCs "shared" only by
the slice when SNC mode is on.

That's what I've argued to the hardware folks lately, at least.