Re: [PATCH v6 1/7] Documentation: DT: arm: add support for sockets defining package boundaries

From: Sudeep Holla <sudeep.holla@arm.com>
To: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Cc: Mark Rutland <mark.rutland@arm.com>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	"Peter Zijlstra \(Intel\)" <peterz@infradead.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Linus Walleij <linus.walleij@linaro.org>,
	Palmer Dabbelt <palmer@sifive.com>,
	Will Deacon <will.deacon@arm.com>,
	Atish Patra <atish.patra@wdc.com>,
	Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
	linux-riscv@lists.infradead.org, Ingo Molnar <mingo@kernel.org>,
	Rob Herring <robh@kernel.org>, Anup Patel <anup@brainfault.org>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	devicetree@vger.kernel.org, Albert Ou <aou@eecs.berkeley.edu>,
	Rob Herring <robh+dt@kernel.org>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-arm-kernel@lists.infradead.org,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	linux-kernel@vger.kernel.org,
	Jeremy Linton <jeremy.linton@arm.com>,
	"Andrew F. Davis" <afd@ti.com>,
	Otto Sabart <ottosabart@seberm.com>,
	Sudeep Holla <sudeep.holla@arm.com>,
	"David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH v6 1/7] Documentation: DT: arm: add support for sockets defining package boundaries
Date: Fri, 31 May 2019 10:37:43 +0100	[thread overview]
Message-ID: <20190531093743.GB18292@e107155-lin> (raw)
In-Reply-To: <20190530214254.tuxsnyv52a2fyhck@shell.armlinux.org.uk>

On Thu, May 30, 2019 at 10:42:54PM +0100, Russell King - ARM Linux admin wrote:
> On Thu, May 30, 2019 at 12:51:03PM +0100, Morten Rasmussen wrote:
> > On Wed, May 29, 2019 at 07:39:17PM -0400, Andrew F. Davis wrote:
> > > On 5/29/19 5:13 PM, Atish Patra wrote:
> > > >From: Sudeep Holla <sudeep.holla@arm.com>
> > > >
> > > >The current ARM DT topology description provides the operating system
> > > >with a topological view of the system that is based on leaf nodes
> > > >representing either cores or threads (in an SMT system) and a
> > > >hierarchical set of cluster nodes that creates a hierarchical topology
> > > >view of how those cores and threads are grouped.
> > > >
> > > >However this hierarchical representation of clusters does not allow to
> > > >describe what topology level actually represents the physical package or
> > > >the socket boundary, which is a key piece of information to be used by
> > > >an operating system to optimize resource allocation and scheduling.
> > > >
> > >
> > > Are physical package descriptions really needed? What does "socket" imply
> > > that a higher layer "cluster" node grouping does not? It doesn't imply a
> > > different NUMA distance and the definition of "socket" is already not well
> > > defined, is a dual chiplet processor not just a fancy dual "socket" or are
> > > dual "sockets" on a server board "slotket" card, will we need new names for
> > > those too..
> >
> > Socket (or package) just implies what you suggest, a grouping of CPUs
> > based on the physical socket (or package). Some resources might be
> > associated with packages and more importantly socket information is
> > exposed to user-space. At the moment clusters are being exposed to
> > user-space as sockets which is less than ideal for some topologies.
>
> Please point out a 32-bit ARM system that has multiple "socket"s.
>
> As far as I'm aware, all 32-bit systems do not have socketed CPUs
> (modern ARM CPUs are part of a larger SoC), and the CPUs are always
> in one package.
>
> Even the test systems I've seen do not have socketed CPUs.
>

As far as we know, there's none. So we simply have to assume all
those systems are single socket(IOW all CPUs reside inside a single
SoC package) system.

--
Regards,
Sudeep

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