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

Cédric Le Goater [off-list ref] 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 <redacted>
Cc: Srikar Dronamraju <redacted>
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