Re: [PATCH] arch_topology: Make cluster topology span at least SMT CPUs

[Ionela Voinescu <ionela.voinescu@arm.com>]

Hi,

On Tuesday 23 Aug 2022 at 21:05:47 (+0800), Yicong Yang wrote:
> On 2022/8/23 16:06, Ionela Voinescu wrote:
> > Hi Yicong,
> > 
> > On Tuesday 23 Aug 2022 at 15:30:44 (+0800), Yicong Yang wrote:
> >> From: Yicong Yang <yangyicong@hisilicon.com>
> >>
> >> Currently cpu_clustergroup_mask() will return CPU mask if cluster span
> >> more or the same CPUs as cpu_coregroup_mask(). This will result topology
> >> borken on non-Cluster SMT machines.
> > 
> > Might be worth adding here:.. "when building with CONFIG_SCHED_CLUSTER=y"
> > 
> 
> will add this qualifier. thanks.
> 
> >>
> > 
> > I thought I had covered this case, but I think I had considered LLC
> > spanning more than one core (tested on TX2 as well).
> > 
> > So you'd only hit this if LLC and cluster level span the same cores (a
> > single core in this case, for non-cluster), in the presence of SMT.
> > 
> 
> Not sure I understand it correctly but in the below case I think the LLC may span
> the same core with socket?

Ah, I've jumped to conclusions based on the "non-cluster" mention. Does
"non-cluster" just mean that there's no intermediary "cluster" grouping of
CPUs between core and LLC? How does the PPTT table look like, by the
way?

Thanks,
Ionela.

> 
> > Reviewed-by: Ionela Voinescu <ionela.voinescu@arm.com>
> > 
> > Many thanks,
> > Ionela.
> > 
> >> Test with:
> >> qemu-system-aarch64 -enable-kvm -machine virt \
> >>  -net none \
> >>  -cpu host \
> >>  -bios ./QEMU_EFI.fd \
> >>  -m 2G \
> >>  -smp 48,sockets=2,cores=12,threads=2 \
> >>  -kernel $Image \
> >>  -initrd $Rootfs \
> >>  -nographic
> >>  -append "rdinit=init console=ttyAMA0 sched_verbose loglevel=8"
> >>
> >> We'll get below error:
> >> [    3.084568] BUG: arch topology borken
> >> [    3.084570]      the SMT domain not a subset of the CLS domain
> >>
> >> Since cluster is a level higher than SMT, fix this by making cluster
> >> spans at least SMT CPUs.
> >>
> >> Cc: Sudeep Holla <sudeep.holla@arm.com>
> >> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> >> Cc: Ionela Voinescu <ionela.voinescu@arm.com>
> >> Fixes: bfcc4397435d ("arch_topology: Limit span of cpu_clustergroup_mask()")
> >> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
> >> ---
> >>  drivers/base/arch_topology.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
> >> index 0424b59b695e..0056a1273275 100644
> >> --- a/drivers/base/arch_topology.c
> >> +++ b/drivers/base/arch_topology.c
> >> @@ -724,7 +724,7 @@ const struct cpumask *cpu_clustergroup_mask(int cpu)
> >>  	 */
> >>  	if (cpumask_subset(cpu_coregroup_mask(cpu),
> >>  			   &cpu_topology[cpu].cluster_sibling))
> >> -		return get_cpu_mask(cpu);
> >> +		return cpu_smt_mask(cpu);
> >>  
> >>  	return &cpu_topology[cpu].cluster_sibling;
> >>  }
> >> -- 
> >> 2.24.0
> >>
> > 
> > .
> >