Re: [PATCH net-next] net: sch_generic: aviod concurrent reset and enqueue op for lockless qdisc

From: Yunsheng Lin <linyunsheng@huawei.com>
To: Eric Dumazet <eric.dumazet@gmail.com>,
	Cong Wang <xiyou.wangcong@gmail.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>,
	Jiri Pirko <jiri@resnulli.us>,
	"David Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	"Linux Kernel Network Developers" <netdev@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>, <linuxarm@huawei.com>
Subject: Re: [PATCH net-next] net: sch_generic: aviod concurrent reset and enqueue op for lockless qdisc
Date: Fri, 4 Sep 2020 16:10:51 +0800	[thread overview]
Message-ID: <5223daf1-a7a2-5d20-7dde-b5b6284f8e02@huawei.com> (raw)
In-Reply-To: <1c01f9e0-fde4-a8ee-caa3-598738a9a98d@gmail.com>

On 2020/9/3 15:24, Eric Dumazet wrote:
> 
> 
> On 9/2/20 6:14 PM, Yunsheng Lin wrote:
> 
>>
>> It seems semantics for some_qdisc_is_busy() is changed, which does not only do
>> the checking, but also do the reseting?
> 
> Yes, obviously, we would have to rename to a better name.
> 
>>
>> Also, qdisc_reset() could be called multi times for the same qdisc if some_qdisc_is_busy()
>> return true multi times?
> 
> This should not matter, qdisc_reset() can be called multiple times,
> as we also call it from qdisc_destroy() anyway.

How about the below patch, which does not need to change the semantics
for some_qdisc_is_busy() and avoid calling qdisc_reset() multi times?

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 265a61d..ce9031c 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -1131,24 +1131,7 @@ EXPORT_SYMBOL(dev_activate);

 static void qdisc_deactivate(struct Qdisc *qdisc)
 {
-	bool nolock = qdisc->flags & TCQ_F_NOLOCK;
-
-	if (qdisc->flags & TCQ_F_BUILTIN)
-		return;
-	if (test_bit(__QDISC_STATE_DEACTIVATED, &qdisc->state))
-		return;
-
-	if (nolock)
-		spin_lock_bh(&qdisc->seqlock);
-	spin_lock_bh(qdisc_lock(qdisc));
-
 	set_bit(__QDISC_STATE_DEACTIVATED, &qdisc->state);
-
-	qdisc_reset(qdisc);
-
-	spin_unlock_bh(qdisc_lock(qdisc));
-	if (nolock)
-		spin_unlock_bh(&qdisc->seqlock);
 }

 static void dev_deactivate_queue(struct net_device *dev,
@@ -1165,6 +1148,33 @@ static void dev_deactivate_queue(struct net_device *dev,
 	}
 }

+static void dev_reset_qdisc(struct net_device *dev)
+{
+	unsigned int i;
+
+	for (i = 0; i < dev->num_tx_queues; i++) {
+		struct netdev_queue *dev_queue;
+		struct Qdisc *q;
+		bool nolock;
+
+		dev_queue = netdev_get_tx_queue(dev, i);
+		q = dev_queue->qdisc_sleeping;
+		nolock = q->flags & TCQ_F_NOLOCK;
+
+		if (nolock)
+			spin_lock_bh(&q->seqlock);
+
+		spin_lock_bh(qdisc_lock(q));
+
+		qdisc_reset(q);
+
+		spin_unlock_bh(qdisc_lock(q));
+
+		if (nolock)
+			spin_unlock_bh(&q->seqlock);
+	}
+}
+
 static bool some_qdisc_is_busy(struct net_device *dev)
 {
 	unsigned int i;
@@ -1219,6 +1229,9 @@ void dev_deactivate_many(struct list_head *head)
 	 */
 	synchronize_net();

+	list_for_each_entry(dev, head, close_list)
+		dev_reset_qdisc(dev);
+
 	/* Wait for outstanding qdisc_run calls. */
 	list_for_each_entry(dev, head, close_list) {
 		while (some_qdisc_is_busy(dev)) {



> 
> 
