Re: [PATCH v5] topology: make core_mask include at least cluster_siblings

[Darren Hart <darren@os.amperecomputing.com>]

On Fri, Sep 16, 2022 at 03:59:34PM +0800, Yicong Yang wrote:
> On 2022/9/16 1:56, Darren Hart wrote:
> > On Thu, Sep 15, 2022 at 08:01:18PM +0800, Yicong Yang wrote:
> >> Hi Darren,
> >>
> > 
> > Hi Yicong,
> > 
> > ...
> > 
> >>> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
> >>> index 1d6636ebaac5..5497c5ab7318 100644
> >>> --- a/drivers/base/arch_topology.c
> >>> +++ b/drivers/base/arch_topology.c
> >>> @@ -667,6 +667,15 @@ const struct cpumask *cpu_coregroup_mask(int cpu)
> >>>  			core_mask = &cpu_topology[cpu].llc_sibling;
> >>>  	}
> >>>  
> >>> +	/*
> >>> +	 * For systems with no shared cpu-side LLC but with clusters defined,
> >>> +	 * extend core_mask to cluster_siblings. The sched domain builder will
> >>> +	 * then remove MC as redundant with CLS if SCHED_CLUSTER is enabled.
> >>> +	 */
> >>> +	if (IS_ENABLED(CONFIG_SCHED_CLUSTER) &&
> >>> +	    cpumask_subset(core_mask, &cpu_topology[cpu].cluster_sibling))
> >>> +		core_mask = &cpu_topology[cpu].cluster_sibling;
> >>> +
> >>>  	return core_mask;
> >>>  }
> >>>  
> >>
> >> Is this patch still necessary for Ampere after Ionela's patch [1], which
> >> will limit the cluster's span within coregroup's span.
> > 
> > Yes, see:
> > https://lore.kernel.org/lkml/YshYAyEWhE4z%2FKpB@fedora/
> > 
> > Both patches work together to accomplish the desired sched domains for the
> > Ampere Altra family.
> > 
> 
> Thanks for the link. From my understanding, on the Altra machine we'll get
> the following results:
> 
> with your patch alone:
> Scheduler will get a weight of 2 for both CLS and MC level and finally the
> MC domain will be squashed. The lowest domain will be CLS.
> 
> with both your patch and Ionela's:
> CLS will have a weight of 1 and MC will have a weight of 2. CLS won't be
> built and the lowest domain will be MC.
> 
> with Ionela's patch alone:
> Both CLS and MC will have a weight of 1, which is incorrect.
> 
> So your patch is still necessary for Amphere Altra. Then we need to limit
> MC span to DIE/NODE span, according to the scheduler's definition for
> topology level, for the issue below. Maybe something like this:

That seems reasonable.

What isn't clear to me is why qemu is creating a cluster layer with the
description you provide. Why is cluster_siblings being populated?

> 
> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
> index 46cbe4471e78..8ebaba576836 100644
> --- a/drivers/base/arch_topology.c
> +++ b/drivers/base/arch_topology.c
> @@ -713,6 +713,9 @@ const struct cpumask *cpu_coregroup_mask(int cpu)
>             cpumask_subset(core_mask, &cpu_topology[cpu].cluster_sibling))
>                 core_mask = &cpu_topology[cpu].cluster_sibling;
> 
> +       if (cpumask_subset(cpu_cpu_mask(cpu), core_mask))
> +               core_mask = cpu_cpu_mask(cpu);
> +
>         return core_mask;
>  }
> 
> >>
> >> I found an issue that the NUMA domains are not built on qemu with:
> >>
> >> qemu-system-aarch64 \
> >>         -kernel ${Image} \
> >>         -smp 8 \
> >>         -cpu cortex-a72 \
> >>         -m 32G \
> >>         -object memory-backend-ram,id=node0,size=8G \
> >>         -object memory-backend-ram,id=node1,size=8G \
> >>         -object memory-backend-ram,id=node2,size=8G \
> >>         -object memory-backend-ram,id=node3,size=8G \
> >>         -numa node,memdev=node0,cpus=0-1,nodeid=0 \
> >>         -numa node,memdev=node1,cpus=2-3,nodeid=1 \
> >>         -numa node,memdev=node2,cpus=4-5,nodeid=2 \
> >>         -numa node,memdev=node3,cpus=6-7,nodeid=3 \
> >>         -numa dist,src=0,dst=1,val=12 \
> >>         -numa dist,src=0,dst=2,val=20 \
> >>         -numa dist,src=0,dst=3,val=22 \
> >>         -numa dist,src=1,dst=2,val=22 \
> >>         -numa dist,src=1,dst=3,val=24 \
> >>         -numa dist,src=2,dst=3,val=12 \
> >>         -machine virt,iommu=smmuv3 \
> >>         -net none \
> >>         -initrd ${Rootfs} \
> >>         -nographic \
> >>         -bios QEMU_EFI.fd \
> >>         -append "rdinit=/init console=ttyAMA0 earlycon=pl011,0x9000000 sched_verbose loglevel=8"
> >>
> >> I can see the schedule domain build stops at MC level since we reach all the
> >> cpus in the system:
> >>
> >> [    2.141316] CPU0 attaching sched-domain(s):
> >> [    2.142558]  domain-0: span=0-7 level=MC
> >> [    2.145364]   groups: 0:{ span=0 cap=964 }, 1:{ span=1 cap=914 }, 2:{ span=2 cap=921 }, 3:{ span=3 cap=964 }, 4:{ span=4 cap=925 }, 5:{ span=5 cap=964 }, 6:{ span=6 cap=967 }, 7:{ span=7 cap=967 }
> >> [    2.158357] CPU1 attaching sched-domain(s):
> >> [    2.158964]  domain-0: span=0-7 level=MC
> >> [...]
> >>
> >> Without this the NUMA domains are built correctly:
> >>
> > > Without which? My patch, Ionela's patch, or both?
> > 
> 
> Revert your patch only will have below result, sorry for the ambiguous. Before reverting,
> for CPU 0, MC should span 0-1 but with your patch it's extended to 0-7 and the scheduler
> domain build will stop at MC level because it has reached all the CPUs.
> 
> >> [    2.008885] CPU0 attaching sched-domain(s):
> >> [    2.009764]  domain-0: span=0-1 level=MC
> >> [    2.012654]   groups: 0:{ span=0 cap=962 }, 1:{ span=1 cap=925 }
> >> [    2.016532]   domain-1: span=0-3 level=NUMA
> >> [    2.017444]    groups: 0:{ span=0-1 cap=1887 }, 2:{ span=2-3 cap=1871 }
> >> [    2.019354]    domain-2: span=0-5 level=NUMA
> > 
> > I'm not following this topology - what in the description above should result in
> > a domain with span=0-5?
> > 
> 
> It emulates a 3-hop NUMA machine and the NUMA domains will be built according to the
> NUMA distances:
> 
> node   0   1   2   3
>   0:  10  12  20  22
>   1:  12  10  22  24
>   2:  20  22  10  12
>   3:  22  24  12  10
> 
> So for CPU 0 the NUMA domains will look like:
> NUMA domain 0 for local nodes (squashed to MC domain), CPU 0-1
> NUMA domain 1 for nodes within distance 12, CPU 0-3
> NUMA domain 2 for nodes within distance 20, CPU 0-5
> NUMA domain 3 for all the nodes, CPU 0-7
> 

Right, thanks for the explanation.

So the bit that remains unclear to me, is why is cluster_siblings being
populated? Which part of your qemu topology description becomes the CLS layer
during sched domain cosntruction?

-- 
Darren Hart
Ampere Computing / OS and Kernel

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