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

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Date:   Wed, 24 Aug 2022 09:31:24 +0800
From:   Yicong Yang <yangyicong@...wei.com>
To:     Ionela Voinescu <ionela.voinescu@....com>
CC:     <yangyicong@...ilicon.com>, <sudeep.holla@....com>,
        <vincent.guittot@...aro.org>, <linux-kernel@...r.kernel.org>,
        <gregkh@...uxfoundation.org>, <rafael@...nel.org>,
        <21cnbao@...il.com>, <jonathan.cameron@...wei.com>,
        <linuxarm@...wei.com>, <prime.zeng@...wei.com>
Subject: Re: [PATCH] arch_topology: Make cluster topology span at least SMT
 CPUs

On 2022/8/24 1:53, Ionela Voinescu wrote:
> 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@...ilicon.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?
> 

yes non-cluster means machine has no cluster topology. The PPTT is generated by the
qemu according to the cmdline so I didn't really check it. See the demo provided in
the commit.

'-smp 48,sockets=2,cores=12,threads=2' builds a mahine with 2 sockets, one socket has
12 cores and each core has 2 threads. If we want to have a cluster machine we build it
with '-smp 48,sockets=2,clusters=6,cores=2,threads=2', then we'll have 6 clusters in
one socket and 2 cores in each cluster, 2 threads for each core.

Thanks.

> Thanks,
> Ionela.
> 
>>
>>> Reviewed-by: Ionela Voinescu <ionela.voinescu@....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@....com>
>>>> Cc: Vincent Guittot <vincent.guittot@...aro.org>
>>>> Cc: Ionela Voinescu <ionela.voinescu@....com>
>>>> Fixes: bfcc4397435d ("arch_topology: Limit span of cpu_clustergroup_mask()")
>>>> Signed-off-by: Yicong Yang <yangyicong@...ilicon.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
>>>>
>>>
>>> .
>>>
> .
>