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Re: [PATCH] xen/mm: avoid watchdog timeout in dump_numa() on large domains
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Tue, 2 Jun 2026 15:09:27 +0200
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- Cc: Bernhard Kaindl <bernhard.kaindl@xxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Tamas K Lengyel <tamas@xxxxxxxxxxxxx>, Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>, Marcus Granado <marcus.granado@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Tue, 02 Jun 2026 13:09:36 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Tue, Jun 02, 2026 at 01:51:13PM +0200, 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?
At some point a similar change will be needed for per-node memory
claims. While we might refuse just for dump_numa(), the intrusive
changes in memory_exchange() are a requirement for per-node claims
IIRC.
Thanks, Roger.
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