[PATCH v2 net-next] net: link_watch: prevent starvation when processing linkwatch wq

[Yunsheng Lin <linyunsheng@huawei.com>]

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 cpu and rtnl locking starvation problem.

This patch releases the cpu and rtnl lock when link watch worker
has processed a fixed number of netdev' link watch event.

Currently __linkwatch_run_queue is called with rtnl lock, so
enfore it with ASSERT_RTNL();

Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
---
V2: use cond_resched and rtnl_unlock after processing a fixed
    number of events
---
 net/core/link_watch.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/net/core/link_watch.c b/net/core/link_watch.c
index 7f51efb..07eebfb 100644
--- a/net/core/link_watch.c
+++ b/net/core/link_watch.c
@@ -168,9 +168,18 @@ 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);
 
+	ASSERT_RTNL();
+
+	/* 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
@@ -200,6 +209,14 @@ static void __linkwatch_run_queue(int urgent_only)
 		}
 		spin_unlock_irq(&lweventlist_lock);
 		linkwatch_do_dev(dev);
+
+		if (--do_dev < 0) {
+			rtnl_unlock();
+			cond_resched();
+			do_dev = MAX_DO_DEV_PER_LOOP;
+			rtnl_lock();
+		}
+
 		spin_lock_irq(&lweventlist_lock);
 	}
 
-- 
2.8.1