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>

1 files changed, 33 insertions, 7 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index 193eafaabd88..04972a4783e2 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -5192,9 +5192,6 @@ 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);
 
@@ -5202,6 +5199,11 @@ int init_dummy_netdev(struct net_device *dev)
         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 @@ out:
 }
 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);