Messages in this thread | | | Date | Fri, 8 Jul 2022 09:27:04 -0700 | From | Darren Hart <> | Subject | Re: [PATCH v6 17/21] arch_topology: Limit span of cpu_clustergroup_mask() |
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On Fri, Jul 08, 2022 at 09:04:24AM +0100, Sudeep Holla wrote: > 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.
I was wrong here. This match also modifies the coregroup, the MC after this patch is equivalent to the CLS before the patch. The Altra is not negatively impacted here.
> > > > 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.
Agree. I worded that badly. I should have said the Altra presents a PPTT topology that accurately reflects the hardwere. There is no shared cpu-side LLC, and there is an affinity between the DSU pairs which share a snoop filter.
I do think the general assumption that MC shares a cpu-side LLC will continue to present challenges to the Altra topology in terms of ongoing to changes to the code. I don't have a good solution to that at the moment, something I'll continue to think on.
> > > 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
Thanks Sudeep,
-- Darren Hart Ampere Computing / OS and Kernel
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