netdev - [PATCH v2] neighbour: Don't let neigh_forced

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Message-ID: <20231206033913.1290566-1-judyhsiao@chromium.org>
Date: Wed,  6 Dec 2023 03:38:33 +0000
From: Judy Hsiao <judyhsiao@...omium.org>
To: Eric Dumazet <edumazet@...gle.com>,
	David Ahern <dsahern@...nel.org>,
	Simon Horman <horms@...nel.org>
Cc: Douglas Anderson <dianders@...omium.org>,
	Judy Hsiao <judyhsiao@...omium.org>,
	Brian Haley <haleyb.dev@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>,
	Joel Granados <joel.granados@...il.com>,
	Julian Anastasov <ja@....bg>,
	Leon Romanovsky <leon@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: [PATCH v2] neighbour: Don't let neigh_forced_gc() disable preemption for long

We are seeing cases where neigh_cleanup_and_release() is called by
neigh_forced_gc() many times in a row with preemption turned off.
When running on a low powered CPU at a low CPU frequency, this has
been measured to keep preemption off for ~10 ms. That's not great on a
system with HZ=1000 which expects tasks to be able to schedule in
with ~1ms latency.

Suggested-by: Douglas Anderson <dianders@...omium.org>
Signed-off-by: Judy Hsiao <judyhsiao@...omium.org>

---

Changes in v2:
- Use ktime_get_ns() for timeout calculation instead of jiffies.

 net/core/neighbour.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index df81c1f0a570..552719c3bbc3 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -253,9 +253,11 @@ static int neigh_forced_gc(struct neigh_table *tbl)
 {
 	int max_clean = atomic_read(&tbl->gc_entries) -
 			READ_ONCE(tbl->gc_thresh2);
+	u64 tmax = ktime_get_ns() + NSEC_PER_MSEC;
 	unsigned long tref = jiffies - 5 * HZ;
 	struct neighbour *n, *tmp;
 	int shrunk = 0;
+	int loop = 0;
 
 	NEIGH_CACHE_STAT_INC(tbl, forced_gc_runs);
 
@@ -278,11 +280,16 @@ static int neigh_forced_gc(struct neigh_table *tbl)
 				shrunk++;
 			if (shrunk >= max_clean)
 				break;
+			if (++loop == 16) {
+				if (ktime_get_ns() > tmax)
+					goto unlock;
+				loop = 0;
+			}
 		}
 	}
 
 	WRITE_ONCE(tbl->last_flush, jiffies);
-
+unlock:
 	write_unlock_bh(&tbl->lock);
 
 	return shrunk;
-- 
2.43.0.rc2.451.g8631bc7472-goog