On 2019/5/31 17:54, Salil Mehta wrote:

quoted

From: netdev-owner@vger.kernel.org On Behalf Of Yunsheng Lin
Sent: Friday, May 31, 2019 10:01 AM
To: davem@davemloft.net
Cc: hkallweit1@gmail.com; f.fainelli@gmail.com;
stephen@networkplumber.org; netdev@vger.kernel.org; linux-
kernel@vger.kernel.org; Linuxarm [off-list ref]
Subject: [PATCH v2 net-next] net: link_watch: prevent starvation when
processing linkwatch wq

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();

Typo enfore --> enforce ?

My mistake.

thanks.

quoted

Signed-off-by: Yunsheng Lin <redacted>
---
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);
+

A comment like below would be helpful in explaining the reason of the code.
 
/* This function is called with rtnl_lock held. If excessive events
 * are present as part of the watch list, their processing could
 * monopolize the rtnl_lock and which could lead to starvation in
 * other modules which want to acquire this lock. Hence, co-operative
 * scheme like below might be helpful in mitigating the problem.
 * This also tries to be fair CPU wise by conditional rescheduling.
 */

Yes, thanks for the helpful comment.

quoted

+		if (--do_dev < 0) {
+			rtnl_unlock();
+			cond_resched();
+			do_dev = MAX_DO_DEV_PER_LOOP;
+			rtnl_lock();
+		}
+
 		spin_lock_irq(&lweventlist_lock);
 	}

.