[PATCH v3 net-next] net: neigh: avoid calling neigh_forced_gc on every alloc when table is full

[Vimal Agrawal <avimalin@gmail.com>]

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 50 ms, but only when gc_thresh3 is configured
at or above NEIGH_FORCED_GC_LARGE_TABLE_THRESH (16384). This avoids
repeated full table scans and lock acquisitions on the hot allocation path
while leaving default-sized and test deployments (gc_thresh3 < 16384)
completely unaffected.

Profiling of neigh_create() shows ~3 orders of magnitude latency
improvement with this change.

Link: https://lore.kernel.org/netdev/CALkUMdSCpx_ywYCx_ePLdm6yioO1nQWx7sSM=AEgsq0kywHxTw@mail.gmail.com/
Signed-off-by: Vimal Agrawal <vimal.agrawal@sophos.com>

---
Thank you for catching this. The v2 rate limit was unconditional, so it
fired even in the selftest which sets gc_thresh3=10, preventing
forced_gc_runs from being incremented.

v3 restricts rate limiting to tables where gc_thresh3 >= 16384. Selftests
and default deployments (gc_thresh3=1024) are unaffected.

v3: Restrict rate limiting to tables with gc_thresh3 >= 16384 to avoid
    breaking selftests and default deployments (gc_thresh3=1024).
v2: Changed rate-limit window from 1s (HZ) to 50ms (msecs_to_jiffies(50))
    based on profiling data showing 44% -> 2.56% CPU reduction.

 net/core/neighbour.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 1349c0eed..9438c5821 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -250,6 +250,8 @@ bool neigh_remove_one(struct neighbour *n)
 	return retval;
 }
 
+#define NEIGH_FORCED_GC_LARGE_TABLE_THRESH    16384
+
 static int neigh_forced_gc(struct neigh_table *tbl)
 {
 	int max_clean = atomic_read(&tbl->gc_entries) -
@@ -260,6 +262,15 @@ static int neigh_forced_gc(struct neigh_table *tbl)
 	int shrunk = 0;
 	int loop = 0;
 
+	/*
+	 * For large neighbor tables, repeated forced GC passes can spend
+	 * significant CPU scanning neighbor entries when most remain active.
+	 * Rate-limit consecutive forced GC passes to reduce CPU overhead.
+	 */
+	if (READ_ONCE(tbl->gc_thresh3) >= NEIGH_FORCED_GC_LARGE_TABLE_THRESH &&
+	    time_before(jiffies, READ_ONCE(tbl->last_flush) + msecs_to_jiffies(50)))
+		return 0;
+
 	NEIGH_CACHE_STAT_INC(tbl, forced_gc_runs);
 
 	spin_lock_bh(&tbl->lock);
-- 
2.43.0