Messages in this thread | | | Date | Fri, 8 Jul 2022 09:04:24 +0100 | From | Sudeep Holla <> | Subject | Re: [PATCH v6 17/21] arch_topology: Limit span of cpu_clustergroup_mask() |
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Hi Darren,
I will let Ionela or Dietmar cover some of the scheduler aspects as I don't have much knowledge in that area.
On Thu, Jul 07, 2022 at 05:10:19PM -0700, Darren Hart wrote: > On Mon, Jul 04, 2022 at 11:16:01AM +0100, Sudeep Holla wrote: > > From: Ionela Voinescu <ionela.voinescu@arm.com> > > Hi Sudeep and Ionela, > > > > > Currently the cluster identifier is not set on DT based platforms. > > The reset or default value is -1 for all the CPUs. Once we assign the > > cluster identifier values correctly, the cluster_sibling mask will be > > populated and returned by cpu_clustergroup_mask() to contribute in the > > creation of the CLS scheduling domain level, if SCHED_CLUSTER is > > enabled. > > > > To avoid topologies that will result in questionable or incorrect > > scheduling domains, impose restrictions regarding the span of clusters, > > Can you provide a specific example of a valid topology that results in > the wrong thing currently? >
As a simple example, Juno with 2 clusters and L2 for each cluster. IIUC MC is preferred instead of CLS and both MC and CLS domains are exact match.
> > > > While previously the scheduling domain builder code would have removed MC > > as redundant and kept CLS if SCHED_CLUSTER was enabled and the > > cpu_coregroup_mask() and cpu_clustergroup_mask() spanned the same CPUs, > > now CLS will be removed and MC kept. > > > > This is not desireable for all systems, particular those which don't > have an L3 but do share other resources - such as the snoop filter in > the case of the Ampere Altra. > > While not universally supported, we agreed in the discussion on the > above patch to allow systems to define clusters independently from the > L3 as an LLC since this is also independently defined in PPTT. > > Going back to my first comment - does this fix an existing system with a > valid topology?
Yes as mentioned above Juno.
> It's not clear to me what that would look like. The Ampere Altra presents > a cluster level in PPTT because that is the desireable topology for the > system.
Absolutely wrong reason. It should present because the hardware is so, not because some OSPM desires something in someway. Sorry that's not how DT/ACPI is designed for. If 2 different OSPM desires different things, then one ACPI will not be sufficient.
> If it's not desirable for another system to have the cluster topology - > shouldn't it not present that layer to the kernel in the first place?
Absolutely 100% yes, it must present it if the hardware is designed so. No if or but.
-- Regards, Sudeep
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