Re: [PATCH v2] mm/page_alloc: only skip kswapd nodes below their kswapd watermark

["Jose A. Perez de Azpillaga" <azpijr@gmail.com>]

On 11/10/26 11:44 PM, Joshua Hahn wrote:
> Commit 19c5fb83f2a4 ("mm: page_alloc: avoid kswapd thrashing due to
> NUMA restrictions") makes get_page_from_freelist() skip any node with an
> active kswapd on the first pass through the zonelist, so unrestricted
> allocations don't immediately consume the memory kswapd is freeing.
> 
> But kswapd is also woken for high-order requests. With THP defrag set to
> defer, each THP fault that fails on a fragmented node wakes kswapd at
> PMD order. kswapd reclaims a batch, wakes kcompactd, and sleeps until
> the next failed fault, so it stays awake much of the time even on a node
> with plenty of free order-0 memory. This means those order-0 allocations
> get directed elsewhere.
> 
> This was observed on some Meta production workloads running on 2-NUMA
> systems, where processes with a preference for node0 allocations
> continuously allocated order-0 pages on node1, despite node0 having
> hundreds of GB of free memory.
> 
> Only skip a zone with an active kswapd if it is also below kswapd's
> target watermark.
> 
> On a 2-NUMA VM with a 60G node0 and a 20G node1, with THP enabled=always
> and defrag=defer, I ran a process that prefers allocating on node0.
> With a fragmented node0:
> - Before the patch, 68% of pages were served from node0
> - After the patch, 100% of pages were served from node0
> 
> One remaining gap is that since this uses kswapd's own threshold,
> unrestricted allocations can take what kswapd frees above the target
> watermark. The current code already lets such allocations drain from the
> high to the low watermark while kswapd sleeps.
> 
> One potential solution, as hinted by Johannes in the discussion from the
> original fix [1] is to add some kind of fudge factor (for instance,
> wmark_pages + high_wmark_pages - low_wmark_pages).
> 
> But in tests that I ran in a VM where the allocation demand outpaces
> kswapd, about 4% more pages landed on the busy node and kswapd reclaimed
> about 15% more, but kswapd still finished and slept. So let's hold off
> on the fudge factor until it becomes an observable issue in workloads.
> 
> [1] https://lore.kernel.org/linux-mm/20251001164131.GB1597553@cmpxchg.org/
> 
> Fixes: 19c5fb83f2a4 ("mm: page_alloc: avoid kswapd thrashing due to NUMA restrictions")
> Suggested-by: Daniel Hodges <hodgesd@meta.com>
> Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
LGTM:

Reviewed-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>

-- 
cheers,
jose a. p-a