Re: [PATCH v1] neighbour: Don't let neigh_forced_gc() disable preemption for long

[Eric Dumazet]

On Fri, Dec 1, 2023 at 9:39 AM Judy Hsiao <judyhsiao@xxxxxxxxxxxx> wrote:
>
> 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.

This will not work in general, because this code runs with BH blocked.

jiffies will stay untouched for many more ms on systems with only one CPU.

I would rather not rely on jiffies here but ktime_get_ns() [1]

Also if we break the loop based on time, we might be unable to purge
the last elements in gc_list.
We might need to use a second list to make sure to cycle over all
elements eventually.


[1]
diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index df81c1f0a57047e176b7c7e4809d2dae59ba6be5..e2340e6b07735db8cf6e75d23ef09bb4b0db53b4
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);

@@ -279,10 +281,16 @@ static int neigh_forced_gc(struct neigh_table *tbl)
                        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;