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From:   Judy Hsiao <judyhsiao@chromium.org>
To:     Eric Dumazet <edumazet@google.com>,
        David Ahern <dsahern@kernel.org>,
        Simon Horman <horms@kernel.org>
Cc:     Douglas Anderson <dianders@chromium.org>,
        Judy Hsiao <judyhsiao@chromium.org>,
        Brian Haley <haleyb.dev@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Joel Granados <joel.granados@gmail.com>,
        Julian Anastasov <ja@ssi.bg>,
        Leon Romanovsky <leon@kernel.org>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org
Subject: [PATCH v1] neighbour: Don't let neigh_forced_gc() disable preemption for long
Date:   Fri,  1 Dec 2023 08:39:03 +0000
Message-ID: <20231201083926.1817394-1-judyhsiao@chromium.org>
MIME-Version: 1.0
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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@chromium.org>
Signed-off-by: Judy Hsiao <judyhsiao@chromium.org>

---

 net/core/neighbour.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index df81c1f0a570..f7a89c7a7673 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -256,6 +256,8 @@ static int neigh_forced_gc(struct neigh_table *tbl)
 	unsigned long tref = jiffies - 5 * HZ;
 	struct neighbour *n, *tmp;
 	int shrunk = 0;
+	bool finish = true;
+	unsigned long timeout = jiffies + msecs_to_jiffies(1);        /* timeout in 1ms */
 
 	NEIGH_CACHE_STAT_INC(tbl, forced_gc_runs);
 
@@ -278,10 +280,14 @@ static int neigh_forced_gc(struct neigh_table *tbl)
 				shrunk++;
 			if (shrunk >= max_clean)
 				break;
+			if (time_after(jiffies, timeout)) {
+				finish = false;
+				break;
+			}
 		}
 	}
-
-	WRITE_ONCE(tbl->last_flush, jiffies);
+	if (finish)
+		WRITE_ONCE(tbl->last_flush, jiffies);
 
 	write_unlock_bh(&tbl->lock);
 
-- 
2.43.0.rc2.451.g8631bc7472-goog