net: percpu net_device refcount

We tried very hard to remove all possible dev_hold()/dev_put() pairs in
network stack, using RCU conversions.

There is still an unavoidable device refcount change for every dst we
create/destroy, and this can slow down some workloads (routers or some
app servers, mmap af_packet)

We can switch to a percpu refcount implementation, now dynamic per_cpu
infrastructure is mature. On a 64 cpus machine, this consumes 256 bytes
per device.

On x86, dev_hold(dev) code :

before
        lock    incl 0x280(%ebx)
after:
        movl    0x260(%ebx),%eax
        incl    fs:(%eax)

Stress bench :

(Sending 160.000.000 UDP frames,
IP route cache disabled, dual E5540 @2.53GHz,
32bit kernel, FIB_TRIE)

Before:

real    1m1.662s
user    0m14.373s
sys     12m55.960s

After:

real    0m51.179s
user    0m15.329s
sys     10m15.942s

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/infiniband/hw/nes/nes_cm.c

@@ -2701,7 +2701,7 @@ static int nes_disconnect(struct nes_qp *nesqp, int abrupt)
 	nesibdev = nesvnic->nesibdev;
 
 	nes_debug(NES_DBG_CM, "netdev refcnt = %u.\n",
-			atomic_read(&nesvnic->netdev->refcnt));
+			netdev_refcnt_read(nesvnic->netdev));
 
 	if (nesqp->active_conn) {
 
@@ -2791,7 +2791,7 @@ int nes_accept(struct iw_cm_id *cm_id, struct iw_cm_conn_param *conn_param)
 	atomic_inc(&cm_accepts);
 
 	nes_debug(NES_DBG_CM, "netdev refcnt = %u.\n",
-			atomic_read(&nesvnic->netdev->refcnt));
+			netdev_refcnt_read(nesvnic->netdev));
 
 	/* allocate the ietf frame and space for private data */
 	nesqp->ietf_frame = pci_alloc_consistent(nesdev->pcidev,

drivers/infiniband/hw/nes/nes_verbs.c

@@ -785,7 +785,7 @@ static struct ib_pd *nes_alloc_pd(struct ib_device *ibdev,
 
 	nes_debug(NES_DBG_PD, "nesvnic=%p, netdev=%p %s, ibdev=%p, context=%p, netdev refcnt=%u\n",
 			nesvnic, nesdev->netdev[0], nesdev->netdev[0]->name, ibdev, context,
-			atomic_read(&nesvnic->netdev->refcnt));
+			netdev_refcnt_read(nesvnic->netdev));
 
 	err = nes_alloc_resource(nesadapter, nesadapter->allocated_pds,
 			nesadapter->max_pd, &pd_num, &nesadapter->next_pd);
@@ -1416,7 +1416,7 @@ static struct ib_qp *nes_create_qp(struct ib_pd *ibpd,
 	/* update the QP table */
 	nesdev->nesadapter->qp_table[nesqp->hwqp.qp_id-NES_FIRST_QPN] = nesqp;
 	nes_debug(NES_DBG_QP, "netdev refcnt=%u\n",
-			atomic_read(&nesvnic->netdev->refcnt));
+			netdev_refcnt_read(nesvnic->netdev));
 
 	return &nesqp->ibqp;
 }

include/linux/netdevice.h

@@ -1026,7 +1026,7 @@ struct net_device {
 	struct timer_list	watchdog_timer;
 
 	/* Number of references to this device */
-	atomic_t		refcnt ____cacheline_aligned_in_smp;
+	int __percpu		*pcpu_refcnt;
 
 	/* delayed register/unregister */
 	struct list_head	todo_list;
@@ -1330,6 +1330,7 @@ static inline void unregister_netdevice(struct net_device *dev)
 	unregister_netdevice_queue(dev, NULL);
 }
 
+extern int 		netdev_refcnt_read(const struct net_device *dev);
 extern void		free_netdev(struct net_device *dev);
 extern void		synchronize_net(void);
 extern int 		register_netdevice_notifier(struct notifier_block *nb);
@@ -1798,7 +1799,7 @@ extern void netdev_run_todo(void);
  */
 static inline void dev_put(struct net_device *dev)
 {
-	atomic_dec(&dev->refcnt);
+	irqsafe_cpu_dec(*dev->pcpu_refcnt);
 }
 
 /**
@@ -1809,7 +1810,7 @@ static inline void dev_put(struct net_device *dev)
  */
 static inline void dev_hold(struct net_device *dev)
 {
-	atomic_inc(&dev->refcnt);
+	irqsafe_cpu_inc(*dev->pcpu_refcnt);
 }
 
 /* Carrier loss detection, dial on demand. The functions netif_carrier_on

net/core/dev.c

@@ -5192,16 +5192,18 @@ int init_dummy_netdev(struct net_device *dev)
 	 */
 	dev->reg_state = NETREG_DUMMY;
 
-	/* initialize the ref count */
-	atomic_set(&dev->refcnt, 1);
-
 	/* NAPI wants this */
 	INIT_LIST_HEAD(&dev->napi_list);
 
 	/* a dummy interface is started by default */
 	set_bit(__LINK_STATE_PRESENT, &dev->state);
 	set_bit(__LINK_STATE_START, &dev->state);
 
+	/* Note : We dont allocate pcpu_refcnt for dummy devices,
+	 * because users of this 'device' dont need to change
+	 * its refcount.
+	 */
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(init_dummy_netdev);
@@ -5243,6 +5245,16 @@ int register_netdev(struct net_device *dev)
 }
 EXPORT_SYMBOL(register_netdev);
 
+int netdev_refcnt_read(const struct net_device *dev)
+{
+	int i, refcnt = 0;
+
+	for_each_possible_cpu(i)
+		refcnt += *per_cpu_ptr(dev->pcpu_refcnt, i);
+	return refcnt;
+}
+EXPORT_SYMBOL(netdev_refcnt_read);
+
 /*
  * netdev_wait_allrefs - wait until all references are gone.
  *
@@ -5257,11 +5269,14 @@ EXPORT_SYMBOL(register_netdev);
 static void netdev_wait_allrefs(struct net_device *dev)
 {
 	unsigned long rebroadcast_time, warning_time;
+	int refcnt;
 
 	linkwatch_forget_dev(dev);
 
 	rebroadcast_time = warning_time = jiffies;
-	while (atomic_read(&dev->refcnt) != 0) {
+	refcnt = netdev_refcnt_read(dev);
+
+	while (refcnt != 0) {
 		if (time_after(jiffies, rebroadcast_time + 1 * HZ)) {
 			rtnl_lock();
 
@@ -5288,11 +5303,13 @@ static void netdev_wait_allrefs(struct net_device *dev)
 
 		msleep(250);
 
+		refcnt = netdev_refcnt_read(dev);
+
 		if (time_after(jiffies, warning_time + 10 * HZ)) {
 			printk(KERN_EMERG "unregister_netdevice: "
 			       "waiting for %s to become free. Usage "
 			       "count = %d\n",
-			       dev->name, atomic_read(&dev->refcnt));
+			       dev->name, refcnt);
 			warning_time = jiffies;
 		}
 	}
@@ -5350,7 +5367,7 @@ void netdev_run_todo(void)
 		netdev_wait_allrefs(dev);
 
 		/* paranoia */
-		BUG_ON(atomic_read(&dev->refcnt));
+		BUG_ON(netdev_refcnt_read(dev));
 		WARN_ON(rcu_dereference_raw(dev->ip_ptr));
 		WARN_ON(dev->ip6_ptr);
 		WARN_ON(dev->dn_ptr);
@@ -5520,9 +5537,13 @@ struct net_device *alloc_netdev_mq(int sizeof_priv, const char *name,
 	dev = PTR_ALIGN(p, NETDEV_ALIGN);
 	dev->padded = (char *)dev - (char *)p;
 
-	if (dev_addr_init(dev))
+	dev->pcpu_refcnt = alloc_percpu(int);
+	if (!dev->pcpu_refcnt)
 		goto free_tx;
 
+	if (dev_addr_init(dev))
+		goto free_pcpu;
+
 	dev_mc_init(dev);
 	dev_uc_init(dev);
 
@@ -5553,6 +5574,8 @@ struct net_device *alloc_netdev_mq(int sizeof_priv, const char *name,
 
 free_tx:
 	kfree(tx);
+free_pcpu:
+	free_percpu(dev->pcpu_refcnt);
 free_p:
 	kfree(p);
 	return NULL;
@@ -5586,6 +5609,9 @@ void free_netdev(struct net_device *dev)
 	list_for_each_entry_safe(p, n, &dev->napi_list, dev_list)
 		netif_napi_del(p);
 
+	free_percpu(dev->pcpu_refcnt);
+	dev->pcpu_refcnt = NULL;
+
 	/*  Compatibility with error handling in drivers */
 	if (dev->reg_state == NETREG_UNINITIALIZED) {
 		kfree((char *)dev - dev->padded);