Re: [PATCH] powerpc/xive: Do not skip CPU-less nodes when creating the IPIs

[Michael Ellerman <mpe@ellerman.id.au>]

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.

cheers