Re: [PATCH net-next] net: neigh: avoid calling neigh_forced_gc on every alloc when table is full

[Jakub Kicinski <kuba@kernel.org>]

On Thu, 25 Jun 2026 10:20:20 +0000 Vimal Agrawal wrote:
> Once the neighbour table exceeds gc_thresh3, neigh_forced_gc() is called
> on every allocation attempt with no rate limiting. In workloads with mostly
> active/reachable entries, the GC walk traverses a large portion of the
> neighbour table without reclaiming entries, holding tbl->lock for an
> extended period. This causes severe lock contention and allocation
> latencies exceeding 16ms under sustained neighbour creation.
> 
> Add a pre-lock check in neigh_forced_gc() to skip the GC run if one was
> performed within the last second, avoiding repeated full table scans and
> lock acquisitions on the hot allocation path.
> 
> Profiling of neigh_create() shows ~3 orders of magnitude latency
> improvement with this change.

I'm not an expert on neigh but 1 second seems a little aggressive.
Can you see if 10msec doesn't give us a similar win?

>  net/core/neighbour.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/net/core/neighbour.c b/net/core/neighbour.c
> index 1349c0eedb64..078842db3c5f 100644
> --- a/net/core/neighbour.c
> +++ b/net/core/neighbour.c
> @@ -260,6 +260,9 @@ static int neigh_forced_gc(struct neigh_table *tbl)
>  	int shrunk = 0;
>  	int loop = 0;
>  
> +	if (!time_after(jiffies, READ_ONCE(tbl->last_flush) + HZ))
> +		return 0;
> +
>  	NEIGH_CACHE_STAT_INC(tbl, forced_gc_runs);
>  
>  	spin_lock_bh(&tbl->lock);