Patch "neighbour: Don't let neigh_forced_gc() disable preemption for long" has been added to the 5.4-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    neighbour: Don't let neigh_forced_gc() disable preemption for long

to the 5.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     neighbour-don-t-let-neigh_forced_gc-disable-preempti.patch
and it can be found in the queue-5.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 77e4a65d88710e70b7a3c5d01f530bdb91be6da8
Author: Judy Hsiao <judyhsiao@xxxxxxxxxxxx>
Date:   Wed Dec 6 03:38:33 2023 +0000

    neighbour: Don't let neigh_forced_gc() disable preemption for long
    
    [ 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@xxxxxxxxxxxx>
    Signed-off-by: Judy Hsiao <judyhsiao@xxxxxxxxxxxx>
    Reviewed-by: David Ahern <dsahern@xxxxxxxxxx>
    Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
    Reviewed-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
    Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 9d631b7adb7b..e571007d083c 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;