linux-kernel - Re: [PATCH] arch_topology: Limit threads to one specific cluster

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Date:   Tue, 24 May 2022 09:51:57 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Gavin Shan <gshan@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        rafael@...nel.org, Jonathan.Cameron@...wei.com, drjones@...hat.com,
        zhenyzha@...hat.com, shan.gavin@...il.com
Subject: Re: [PATCH] arch_topology: Limit threads to one specific cluster

On Tue, May 24, 2022 at 04:12:12PM +0800, Gavin Shan wrote:
> The sibling information for one particular CPU is updated after ACPI
> PPTT table is parsed. struct cpu_topology::thread_sibling tracks the
> the CPUs in same core. However, cluster isn't considered when it's
> populated. In this case, multiple threads belonging to different
> clusters can be put together through the sibling information. It
> eventually leads to unexpected warning from sched subsystem.
> 
> For example, the following warning is observed in a VM where we have
> 2 sockets, 4 clusters, 8 cores and 16 threads and the CPU topology
> is populated as below.
> 
>    CPU  Socket-ID  Cluster-ID  Core-ID  Thread-ID
>    ----------------------------------------------
>     0      0          0          0        0
>     1      0          0          0        1
>     2      0          0          1        0
>     3      0          0          1        1
>     4      0          1          0        0
>     5      0          1          0        1
>     6      0          1          1        0
>     7      0          1          1        1
>     8      1          0          0        0
>     9      1          0          0        1
>    10      1          0          1        0
>    11      1          0          1        1
>    12      1          1          0        0
>    13      1          1          0        1
>    14      1          1          1        0
>    15      1          1          1        1
> 
>   [    0.592181] CPU: All CPU(s) started at EL1
>   [    0.593766] alternatives: patching kernel code
>   [    0.595890] BUG: arch topology borken
>   [    0.597210]      the SMT domain not a subset of the CLS domain
>   [    0.599286]      child=0-1,4-5    sd=0-3
> 
>   # cat /sys/devices/system/cpu/cpu0/topology/cluster_cpus_list
>   0-3
>   # cat /sys/devices/system/cpu/cpu0/topology/thread_siblings_list
>   0-1,4-5
>
> This fixes the issue by limiting threads to one specific cluster.
> With this applied, the unexpected warning disappears in the VM.
>

I have similar fix but as part of bigger series[1] to get DT support in
line with ACPI.

-- 
Regards,
Sudeep

[1] https://lore.kernel.org/lkml/20220518093325.2070336-3-sudeep.holla@arm.com