From mboxrd@z Thu Jan 1 00:00:00 1970 From: Morten Rasmussen Subject: Re: [PATCH v6 1/7] Documentation: DT: arm: add support for sockets defining package boundaries Date: Thu, 30 May 2019 14:12:41 +0100 Message-ID: <20190530131241.GB10919@e105550-lin.cambridge.arm.com> References: <20190529211340.17087-1-atish.patra@wdc.com> <20190529211340.17087-2-atish.patra@wdc.com> <49f41e62-5354-a674-d95f-5f63851a0ca6@ti.com> <20190530115103.GA10919@e105550-lin.cambridge.arm.com> <70639181-09d1-4644-f062-b19e06db7471@ti.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <70639181-09d1-4644-f062-b19e06db7471@ti.com> Sender: linux-kernel-owner@vger.kernel.org To: "Andrew F. Davis" Cc: Mark Rutland , "Rafael J. Wysocki" , "Peter Zijlstra (Intel)" , Catalin Marinas , Linus Walleij , Palmer Dabbelt , Will Deacon , Atish Patra , Mauro Carvalho Chehab , linux-riscv@lists.infradead.org, Ingo Molnar , Rob Herring , Anup Patel , Russell King , devicetree@vger.kernel.org, Albert Ou , Rob Herring , Paul Walmsley , Thomas Gleixner , linux-arm-kernel@lists.infradead.org List-Id: devicetree@vger.kernel.org On Thu, May 30, 2019 at 08:56:03AM -0400, Andrew F. Davis wrote: > On 5/30/19 7:51 AM, 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 > >>> > >>>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. > > > > I see the benefit of reporting the physical layout and packaging information > to user-space for tracking reasons, but from software perspective this > doesn't matter, and the resource partitioning should be described elsewhere > (NUMA nodes being the go to example). That would make defining a NUMA node mandatory even for non-NUMA systems? > >At the moment user-space is only told about hw threads, cores, and > >sockets. In the very near future it is going to be told about dies too > >(look for Len Brown's multi-die patch set). > > > > Seems my hypothetical case is already in the works :( Indeed. IIUC, the reasoning behind it is related to actual multi-die x86 packages and some rapl stuff being per-die or per-core. > > >I don't see how we can provide correct information to user-space based > >on the current information in DT. I'm not convinced it was a good idea > >to expose this information to user-space to begin with but that is > >another discussion. > > > > Fair enough, it's a little late now to un-expose this info to userspace so > we should at least present it correctly. My worry was this getting out of > hand with layering, for instance what happens when we need to add die nodes > in-between cluster and socket? If we want the die mask to be correct for arm/arm64/riscv we need die information from somewhere. I'm not in favour of adding more topology layers to the user-space visible topology description, but others might have a valid reason and if it is exposed I would prefer if we try to expose the right information. Btw, for packages, we already have that information in ACPI/PPTT so it would be nice if we could have that for DT based systems too. Morten