Re: [PATCH v3] topology: make core_mask include at least cluster_siblings

From: Darren Hart <darren@os.amperecomputing.com>
To: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>,
	Will Deacon <will@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Linux Arm <linux-arm-kernel@lists.infradead.org>,
	Sudeep Holla <sudeep.holla@arm.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Catalin Marinas <Catalin.Marinas@arm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Valentin Schneider <Valentin.Schneider@arm.com>,
	"D . Scott Phillips" <scott@os.amperecomputing.com>,
	Ilkka Koskinen <ilkka@os.amperecomputing.com>,
	stable@vger.kernel.org, Barry Song <21cnbao@gmail.com>
Subject: Re: [PATCH v3] topology: make core_mask include at least cluster_siblings
Date: Wed, 16 Mar 2022 08:20:32 -0700	[thread overview]
Message-ID: <YjIAQKwfa3/vr/kU@fedora> (raw)
In-Reply-To: <eb33745a-9d63-89b1-1245-9d1e0e04a169@arm.com>

On Wed, Mar 16, 2022 at 03:48:50PM +0100, Dietmar Eggemann wrote:
> - Barry Song <song.bao.hua@hisilicon.com> (always get undelivered mail
>   returned to sender)
> + Barry Song <21cnbao@gmail.com>
> 
> On 14/03/2022 17:54, Darren Hart wrote:
> > On Mon, Mar 14, 2022 at 05:35:05PM +0100, Dietmar Eggemann wrote:
> >> On 09/03/2022 19:26, Darren Hart wrote:
> >>> On Wed, Mar 09, 2022 at 01:50:07PM +0100, Dietmar Eggemann wrote:
> >>>> On 08/03/2022 18:49, Darren Hart wrote:
> >>>>> On Tue, Mar 08, 2022 at 05:03:07PM +0100, Dietmar Eggemann wrote:
> >>>>>> On 08/03/2022 12:04, Vincent Guittot wrote:
> >>>>>>> On Tue, 8 Mar 2022 at 11:30, Will Deacon <will@kernel.org> wrote:
> 
> [...]
> 
> > Ultimately, this delivers the same result. I do think it imposes more complexity
> > for everyone to address what as far as I'm aware only affect the one system.
> > 
> > I don't think the term "Cluster" has a clear and universally understood
> > definition, so I don't think it's a given that "CLS should be sub-SD of MC". I
> 
> I agree, the term 'cluster' is overloaded but default_topology[] clearly
> says (with direction up means smaller SD spans).
> 
>   #ifdef CONFIG_SCHED_CLUSTER
>         { cpu_clustergroup_mask, cpu_cluster_flags, SD_INIT_NAME(CLS) },
>   #endif
> 
>   #ifdef CONFIG_SCHED_MC
>         { cpu_coregroup_mask, cpu_core_flags, SD_INIT_NAME(MC) },
>   #endif
> 

Right, understood. It is a clear expectation of the current Sched Domain
topology abstraction.

> In ACPI code we have `cluster_node = fetch_pptt_node(... ,
> cpu_node->parent) but then the cache information (via
> llc_id/llc_sibling) can change things which make this less easy to grasp.
> 
> > think this has been assumed, and that assumption has mostly held up, but this is
> > an abstraction, and the abstraction should follow the physical topologies rather
> > than the other way around in my opinion. If that's the primary motivation for
> > this approach, I don't think it justifies the additional complexity.
> > 
> > All told, I prefer the 2 line change contained within cpu_coregroup_mask() which
> > handles the one known exception with minimal impact. It's easy enough to come
> > back to this to address more cases with a more complex solution if needed in the
> > future - but I prefer to introduce the least amount of complexity as possible to
> > address the known issues, especially if the end result is the same and the cost
> > is paid by the affected systems.
> > 
> > Thanks,
> 
> Yeah, I can see your point. It's the smaller hack. My solution just
> prevents us to manipulate the coregroup mask only to get the MC layer
> degenerated by the core topology code. But people might say that's a
> clever thing to do here. So I'm fine with your original solution as well.
> 
> [...]

Thanks Dietmar,

Sudeep, do we have sufficient consensus to pull in this patch?

Thanks,

-- 
Darren Hart
Ampere Computing / OS and Kernel