Re: [PATCH] xen/mm: avoid watchdog timeout in dump_numa() on large domains

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 02/06/2026 12:51 pm, Jan Beulich wrote:
> On 02.06.2026 10:49, Bernhard Kaindl wrote:
>> Using the 'u' debug key invokes dump_numa(), which walks each domain's
>> page list under page_alloc_lock to compute per-NUMA-node counts. On
>> domains with many pages, this O(pages) operation can hold the lock long
>> enough to trigger a watchdog timeout.
> In addition to what Roger said: Is it really the lock holding that's a
> problem here? That is, there would be no problem if there was no lock
> involved in this O(pages) operation?
>
>> Replace the page-list walk with node_tot_pages[], a per-node counter
>> maintained in struct domain. This reduces dump_numa()'s per-domain work
>> from O(pages) to O(nodes).
> Alternative approch for consideration: Purge dump_numa()? This big a
> change for making a keyhandler work better is somewhat questionable an
> approach, imo. The keyhandler isn't there for use in production anyway,
> it's (primarily) a debugging aid. If the data is still needed (and may
> e.g. be useful on production systems), make a (preemptible) domctl or
> sysctl or alike instead?

Introducing d->node_tot_pages[] is a prerequisite for per-node claims. 
Tidying up dump_numa() is just a useful side effect.

>From an upstream perspective, it does not want separating from the
per-node claims work.

~Andrew