[PATCH v3 net-next] net: link_watch: prevent starvation when processing linkwatch wq

[Yunsheng Lin]

When user has configured a large number of virtual netdev, such
as 4K vlans, the carrier on/off operation of the real netdev
will also cause it's virtual netdev's link state to be processed
in linkwatch. Currently, the processing is done in a work queue,
which may cause rtnl locking starvation problem and worker
starvation problem for other work queue, such as irqfd_inject wq.

This patch releases the cpu when link watch worker has processed
a fixed number of netdev' link watch event, and schedule the
work queue again when there is still link watch event remaining.

Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
---
V2: use cond_resched and rtnl_unlock after processing a fixed
    number of events
V3: fall back to v1 and change commit log to reflect that.
---
---
 net/core/link_watch.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/net/core/link_watch.c b/net/core/link_watch.c
index 04fdc95..f153e06 100644
--- a/net/core/link_watch.c
+++ b/net/core/link_watch.c
@@ -163,9 +163,16 @@ static void linkwatch_do_dev(struct net_device *dev)
 
 static void __linkwatch_run_queue(int urgent_only)
 {
+#define MAX_DO_DEV_PER_LOOP	100
+
+	int do_dev = MAX_DO_DEV_PER_LOOP;
 	struct net_device *dev;
 	LIST_HEAD(wrk);
 
+	/* Give urgent case more budget */
+	if (urgent_only)
+		do_dev += MAX_DO_DEV_PER_LOOP;
+
 	/*
 	 * Limit the number of linkwatch events to one
 	 * per second so that a runaway driver does not
@@ -184,7 +191,7 @@ static void __linkwatch_run_queue(int urgent_only)
 	spin_lock_irq(&lweventlist_lock);
 	list_splice_init(&lweventlist, &wrk);
 
-	while (!list_empty(&wrk)) {
+	while (!list_empty(&wrk) && do_dev > 0) {
 
 		dev = list_first_entry(&wrk, struct net_device, link_watch_list);
 		list_del_init(&dev->link_watch_list);
@@ -195,9 +202,13 @@ static void __linkwatch_run_queue(int urgent_only)
 		}
 		spin_unlock_irq(&lweventlist_lock);
 		linkwatch_do_dev(dev);
+		do_dev--;
 		spin_lock_irq(&lweventlist_lock);
 	}
 
+	/* Add the remaining work back to lweventlist */
+	list_splice_init(&wrk, &lweventlist);
+
 	if (!list_empty(&lweventlist))
 		linkwatch_schedule_work(0);
 	spin_unlock_irq(&lweventlist_lock);
-- 
2.8.1