Re: [PATCH] mm/migrate: wake kcompactd when huge folio migration fails in numa balancing

[Joshua Hahn <joshua.hahnjy@gmail.com>]

On Thu, 22 Oct 2026 14:59:17 -0400 "Zi Yan" <ziy@nvidia.com> wrote:

> On Thu Oct 22, 2026 at 1:25 PM EDT, Joshua Hahn wrote:
> > NUMA balancing migrates a misplaced folio if migrate_balanced_pgdat()
> > finds enough headroom on the destination node. For large folios, it then
> > allocates a folio from the destination with GFP_TRANSHUGE_LIGHT, which
> > wakes up neither kswapd nor kcompactd. This means a hot large folio
> > trying to promote to a fragmented destination may never succeed in
> > promotion even after repeated retries.
> >
> > Wake kcompactd on the target node when the destination allocation fails
> > for a large folio. Don't wake kswapd, since this path can only be
> > reached if there is enough free memory to service the allocation, but it
> > is too fragmented to serve the large folio allocation.
> >
> > This patch has been A/B tested on tiered Meta systems (DRAM + CXL)
> > running identical cacheing workloads. Without this patch, we accumulate
> > around 4.5k failed THP migrations over the span of a day, while the host
> > with this patch accumulates only about 80.
> 
> Is proactive compaction enabled? If yes, why does it not help?

Hi Zi, thanks for taking a look!

Yes, indeed proactive compaction is enabled with vm.compaction_proactiveness
set to 20. 

Hmm, I wonder why we are seeing this difference then. 

One reason that I can think of is that kswapd is awake very often. My
understanding is that proactive compaction does not run when kswapd is
on, with the idea being that if we can free memory, compaction becomes
easier in the future, so we should try when kswapd is done.

When they overlap, then proactive reclaim is suppressed, so it could be
helpful to have a manual trigger to wake it up when a high order
allocation fails. What do you think? If it would help, I can add this
detail to the commit message as well.

Thanks again for taking a look, Zi. I hope you have a great day!
Joshua

> > Those migration differences leads to about 25% reduced CXL bandwidth
> > usage after applying this patch.
> >
> > Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
> > ---
> >  mm/migrate.c | 13 ++++++++++++-
> >  1 file changed, 12 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 7bdcdb57652f8..67364fe0851c0 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -2671,6 +2671,7 @@ static struct folio *alloc_misplaced_dst_folio(struct folio *src,
> >  	int nid = (int) data;
> >  	int order = folio_order(src);
> >  	gfp_t gfp = __GFP_THISNODE;
> > +	struct folio *dst;
> >  
> >  	if (order > 0)
> >  		gfp |= GFP_TRANSHUGE_LIGHT;
> > @@ -2679,7 +2680,17 @@ static struct folio *alloc_misplaced_dst_folio(struct folio *src,
> >  			__GFP_NOWARN;
> >  		gfp &= ~__GFP_RECLAIM;
> >  	}
> > -	return __folio_alloc_node(gfp, order, nid);
> > +
> > +	dst = __folio_alloc_node(gfp, order, nid);
> > +
> > +	/*
> > +	 * A node with enough free memory might still be too fragmented to serve
> > +	 * a large folio. Wake kcompactd to help the next huge folio migration.
> > +	 */
> > +	if (!dst && order > 0)
> > +		wakeup_kcompactd(NODE_DATA(nid), order, gfp_zone(gfp));
> > +
> > +	return dst;
> >  }
> >  
> >  /*
> 
> 
> 
> 
> -- 
> Best Regards,
> Yan, Zi