Re: [RFC 0/2] Add RISC-V cpu topology

From: Sudeep Holla <sudeep.holla@arm.com>
To: Nick Kossifidis <mick@ics.forth.gr>
Cc: mark.rutland@arm.com, devicetree@vger.kernel.org,
	Damien.LeMoal@wdc.com, alankao@andestech.com, hch@infradead.org,
	anup@brainfault.org, palmer@sifive.com,
	linux-kernel@vger.kernel.org, zong@andestech.com,
	Atish Patra <atish.patra@wdc.com>,
	robh+dt@kernel.org, Sudeep Holla <sudeep.holla@arm.com>,
	linux-riscv@lists.infradead.org, tglx@linutronix.de
Subject: Re: [RFC 0/2] Add RISC-V cpu topology
Date: Tue, 6 Nov 2018 14:13:49 +0000	[thread overview]
Message-ID: <20181106141331.GA28458@e107155-lin> (raw)
Message-ID: <20181106141349.ERBh93_k2R35OOIptlKgOOBgdouLZGw4iufq1gt72KA@z> (raw)
In-Reply-To: <866dedbc78ab4fa0e3b040697e112106@mailhost.ics.forth.gr>

On Fri, Nov 02, 2018 at 08:58:39PM +0200, Nick Kossifidis wrote:
> Hello All,
> 
> Στις 2018-11-02 01:04, Atish Patra έγραψε:
> > This patch series adds the cpu topology for RISC-V. It contains
> > both the DT binding and actual source code. It has been tested on
> > QEMU & Unleashed board.
> > 
> > The idea is based on cpu-map in ARM with changes related to how
> > we define SMT systems. The reason for adopting a similar approach
> > to ARM as I feel it provides a very clear way of defining the
> > topology compared to parsing cache nodes to figure out which cpus
> > share the same package or core.  I am open to any other idea to
> > implement cpu-topology as well.
> > 
> 
> I was also about to start a discussion about CPU topology on RISC-V
> after the last swtools group meeting. The goal is to provide the
> scheduler with hints on how to distribute tasks more efficiently
> between harts, by populating the scheduling domain topology levels
> (https://elixir.bootlin.com/linux/v4.19/ident/sched_domain_topology_level).
> What we want to do is define cpu groups and assign them to
> scheduling domains with the appropriate SD_ flags
> (https://github.com/torvalds/linux/blob/master/include/linux/sched/topology.h#L16).
>

OK are we defining a CPU topology binding for Linux scheduler ?
NACK for all the approaches that assumes any knowledge of OS scheduler.

> So the cores that belong to a scheduling domain may share:
> CPU capacity (SD_SHARE_CPUCAPACITY / SD_ASYM_CPUCAPACITY)
> Package resources -e.g. caches, units etc- (SD_SHARE_PKG_RESOURCES)
> Power domain (SD_SHARE_POWERDOMAIN)
> 

Too Linux kernel/scheduler specific to be part of $subject

> In this context I believe using words like "core", "package",
> "socket" etc can be misleading. For example the sample topology you
> use on the documentation says that there are 4 cores that are part
> of a package, however "package" has a different meaning to the
> scheduler. Also we don't say anything in case they share a power
> domain or if they have the same capacity or not. This mapping deals
> only with cache hierarchy or other shared resources.
>

{Un,}fortunately those are terms used by hardware people.

> How about defining a dt scheme to describe the scheduler domain
> topology levels instead ? e.g:
>

NACK as already mentioned above.

--
Regards,
Sudeep

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