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

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Date:   Tue, 24 May 2022 11:59:53 +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 05:05:22PM +0800, Gavin Shan wrote:
> Hi Sudeep,
> 
> On 5/24/22 4:51 PM, Sudeep Holla wrote:
> > 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.
> > 
> 
> Your patch resolves the issue I have. So please ignore mine. Sorry

Thanks for that.

> for the noise.
>

No worries, definitely not noise.

-- 
Regards,
Sudeep