netdev - [PATCH AUTOSEL 5.4 4/7] neighbour: Don't let neigh_forced

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Date: Mon, 18 Dec 2023 07:46:49 -0500
From: Sasha Levin <sashal@...nel.org>
To: linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Cc: Judy Hsiao <judyhsiao@...omium.org>,
	Douglas Anderson <dianders@...omium.org>,
	David Ahern <dsahern@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	"David S . Miller" <davem@...emloft.net>,
	Sasha Levin <sashal@...nel.org>,
	kuba@...nel.org,
	pabeni@...hat.com,
	martin.lau@...nel.org,
	ja@....bg,
	joel.granados@...il.com,
	leon@...nel.org,
	haleyb.dev@...il.com,
	netdev@...r.kernel.org
Subject: [PATCH AUTOSEL 5.4 4/7] neighbour: Don't let neigh_forced_gc() disable preemption for long

From: Judy Hsiao <judyhsiao@...omium.org>

[ Upstream commit e5dc5afff62f3e97e86c3643ec9fcad23de4f2d3 ]

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>
Reviewed-by: David Ahern <dsahern@...nel.org>
Reviewed-by: Eric Dumazet <edumazet@...gle.com>
Reviewed-by: Douglas Anderson <dianders@...omium.org>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 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 9d631b7adb7bf..e571007d083cc 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -226,9 +226,11 @@ 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);
 
@@ -251,11 +253,16 @@ 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