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Subject: Re: [PATCH] powerpc/xive: Do not skip CPU-less nodes when creating
 the IPIs
To: Michael Ellerman <mpe@ellerman.id.au>, <linuxppc-dev@lists.ozlabs.org>
References: <20210629131542.743888-1-clg@kaod.org>
 <87a6m0l5by.fsf@mpe.ellerman.id.au>
From: =?UTF-8?Q?C=c3=a9dric_Le_Goater?= <clg@kaod.org>
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Date: Mon, 2 Aug 2021 14:59:01 +0200
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Cc: Laurent Vivier <lvivier@redhat.com>,
 Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
 Geetika Moolchandani <Geetika.Moolchandani1@ibm.com>, stable@vger.kernel.org,
 David Gibson <dgibson@redhat.com>, Kairui Song <kasong@redhat.com>
Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

On 8/2/21 8:37 AM, Michael Ellerman wrote:
> Cédric Le Goater <clg@kaod.org> writes:
>> On PowerVM, CPU-less nodes can be populated with hot-plugged CPUs at
>> runtime. Today, the IPI is not created for such nodes, and hot-plugged
>> CPUs use a bogus IPI, which leads to soft lockups.
>>
>> We could create the node IPI on demand but it is a bit complex because
>> this code would be called under bringup_up() and some IRQ locking is
>> being done. The simplest solution is to create the IPIs for all nodes
>> at startup.
>>
>> Fixes: 7dcc37b3eff9 ("powerpc/xive: Map one IPI interrupt per node")
>> Cc: stable@vger.kernel.org # v5.13
>> Reported-by: Geetika Moolchandani <Geetika.Moolchandani1@ibm.com>
>> Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
>> Signed-off-by: Cédric Le Goater <clg@kaod.org>
>> ---
>>
>> This patch breaks old versions of irqbalance (<= v1.4). Possible nodes
>> are collected from /sys/devices/system/node/ but CPU-less nodes are
>> not listed there. When interrupts are scanned, the link representing
>> the node structure is NULL and segfault occurs.
> 
> Breaking userspace is usually frowned upon, even if it is irqbalance.
>
> If CPU-less nodes appeared in /sys/devices/system/node would that fix
> it? Could we do that or is that not possible for other reasons?
> 
>> Version 1.7 seems immune. 
> 
> Which was released in August 2020.
> 
> Looks like some distros still ship 1.6, I take it you're not sure if
> that is broken or not.

I did a bisect on irqbalance and the "bad" commit was introduced between 
version 1.7 and version 1.8 :

  commit 31dea01f3a47 ("Also fetch node info for non-PCI devices") 
  https://github.com/Irqbalance/irqbalance/commit/31dea01f3a47aa6374560638486879e5129f9c94

which was backported on RHEL 8 in RPM irqbalance-1.4.0-6.el8.

Any distro using irqbalance <= 1.7 without the patch above is fine. 

Since irqbalance handled cleanly irqs referencing offline nodes before 
this patch, I am inclined to think that the irqbalance fix is incomplete. 
Unfortunately, the commit log lacks some context on the non-PCI devices. 

Thanks,

C.