> From: [email protected] [mailto:netdev- > [email protected]] On Behalf Of Yunsheng Lin > Sent: Friday, May 31, 2019 10:01 AM > To: [email protected] > Cc: [email protected]; [email protected]; > [email protected]; [email protected]; linux- > [email protected]; Linuxarm <[email protected]> > 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(); > > Signed-off-by: Yunsheng Lin <[email protected]> > --- > 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();
Sorry, missed in my earlier comment. I could see multiple problems here and please correct me if I am wrong: 1. It looks like releasing the rtnl_lock here and then res-scheduling might not be safe, especially when you have already held *lweventlist_lock* (which is global and not per-netdev), and when you are trying to reschedule. This can cause *deadlock* with itself. Reason: once you release the rtnl_lock() the similar leg of function netdev_wait_allrefs() could be called for some other netdevice which might end up in waiting for same global linkwatch event list lock i.e. *lweventlist_lock*. 2. After releasing the rtnl_lock() we have not ensured that all the rcu operations are complete. Perhaps we need to take rcu_barrier() before retaking the rtnl_lock() > + do_dev = MAX_DO_DEV_PER_LOOP; Here, I think rcu_barrier() should exist. > + rtnl_lock(); > + } > + > spin_lock_irq(&lweventlist_lock); > }
