linux-kernel - Re: [PATCH v5 05/10] powerpc/smp: Dont assume l2-cache to be superset of sibling

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Date:   Sun, 13 Sep 2020 22:23:22 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Nathan Lynch <nathanl@...ux.ibm.com>,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Michael Neuling <mikey@...ling.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jordan Niethe <jniethe5@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Valentin Schneider <valentin.schneider@....com>,
        "Oliver O'Halloran" <oohall@...il.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v5 05/10] powerpc/smp: Dont assume l2-cache to be
 superset of sibling

* Michael Ellerman <mpe@...erman.id.au> [2020-09-13 11:46:41]:

> Srikar Dronamraju <srikar@...ux.vnet.ibm.com> writes:
> > * Michael Ellerman <mpe@...erman.id.au> [2020-09-11 21:55:23]:
> >
> >> Srikar Dronamraju <srikar@...ux.vnet.ibm.com> writes:
> >> > Current code assumes that cpumask of cpus sharing a l2-cache mask will
> >> > always be a superset of cpu_sibling_mask.
> >> >
> >> > Lets stop that assumption. cpu_l2_cache_mask is a superset of
> >> > cpu_sibling_mask if and only if shared_caches is set.
> >> 
> >> I'm seeing oopses with this:
> >> 
> 
> The patch fixes qemu, and I don't see the crash on mambo, but I still
> see:
>   [    0.010536] smp: Bringing up secondary CPUs ...
>   [    0.019189] smp: Brought up 2 nodes, 8 CPUs
>   [    0.019210] numa: Node 0 CPUs: 0-3
>   [    0.019235] numa: Node 1 CPUs: 4-7
>   [    0.024444]      the CACHE domain not a subset of the MC domain
>   [    0.024505] BUG: arch topology borken
>   [    0.024527]      the SMT domain not a subset of the CACHE domain
>   [    0.024563] BUG: arch topology borken
>   [    0.024584]      the CACHE domain not a subset of the MC domain
>   [    0.024645] BUG: arch topology borken
>   [    0.024666]      the SMT domain not a subset of the CACHE domain
>   [    0.024702] BUG: arch topology borken
>   [    0.024723]      the CACHE domain not a subset of the MC domain
> 
> That's the p9 mambo model, using skiboot.tcl from skiboot, with CPUS=2,
> THREADS=4 and MAMBO_NUMA=1.
> 

I was able to reproduce with
 qemu-system-ppc64 -nographic -vga none -M pseries -cpu POWER8 \
   -kernel build~/vmlinux \
   -m 2G,slots=2,maxmem=4G \
   -object memory-backend-ram,size=1G,id=m0 \
   -object memory-backend-ram,size=1G,id=m1 \
   -numa node,nodeid=0,memdev=m0 \
   -numa node,nodeid=1,memdev=m1 \
   -smp 8,threads=4,sockets=2,maxcpus=8  \


If the CPU doesn't have a l2-cache element, then CPU not only has to set
itself in the cpu_l2_cache but also the siblings. Otherwise it will so
happen that the Siblings will have 4 Cpus set, and the Cache domain will
have just one cpu set, leading to this BUG message.

Patch follows this mail.

> Node layout is:
> 
> [    0.000000] Early memory node ranges
> [    0.000000]   node   0: [mem 0x0000000000000000-0x00000000ffffffff]
> [    0.000000]   node   1: [mem 0x0000200000000000-0x00002000ffffffff]
> [    0.000000] Initmem setup node 0 [mem 0x0000000000000000-0x00000000ffffffff]
> [    0.000000] On node 0 totalpages: 65536
> [    0.000000] Initmem setup node 1 [mem 0x0000200000000000-0x00002000ffffffff]
> [    0.000000] On node 1 totalpages: 65536
> 
> 
> There aren't any l2-cache properties in the device-tree under cpus.
> 
> I'll try and have a closer look tonight.
> 
> cheers

-- 
Thanks and Regards
Srikar Dronamraju