net: Use this_cpu_inc() to increment net->core_stats

The macro dev_core_stats_##FIELD##_inc() disables preemption and invokes
netdev_core_stats_alloc() to return a per-CPU pointer.
netdev_core_stats_alloc() will allocate memory on its first invocation
which breaks on PREEMPT_RT because it requires non-atomic context for
memory allocation.

This can be avoided by enabling preemption in netdev_core_stats_alloc()
assuming the caller always disables preemption.

It might be better to replace local_inc() with this_cpu_inc() now that
dev_core_stats_##FIELD##_inc() gained a preempt-disable section and does
not rely on already disabled preemption. This results in less
instructions on x86-64:
local_inc:
|          incl %gs:__preempt_count(%rip)  # __preempt_count
|          movq    488(%rdi), %rax # _1->core_stats, _22
|          testq   %rax, %rax      # _22
|          je      .L585   #,
|          add %gs:this_cpu_off(%rip), %rax        # this_cpu_off, tcp_ptr__
|  .L586:
|          testq   %rax, %rax      # _27
|          je      .L587   #,
|          incq (%rax)            # _6->a.counter
|  .L587:
|          decl %gs:__preempt_count(%rip)  # __preempt_count

this_cpu_inc(), this patch:
|         movq    488(%rdi), %rax # _1->core_stats, _5
|         testq   %rax, %rax      # _5
|         je      .L591   #,
| .L585:
|         incq %gs:(%rax) # _18->rx_dropped

Use unsigned long as type for the counter. Use this_cpu_inc() to
increment the counter. Use a plain read of the counter.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/YmbO0pxgtKpCw4SY@linutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 59e27a2b7bf04a28d98d987dedd46dc11d4b8bc8..b1fbe21650bb5ed1d8593d895ba3106869311074 100644 (file)
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -199,10 +199,10 @@ struct net_device_stats {
  * Try to fit them in a single cache line, for dev_get_stats() sake.
  */
 struct net_device_core_stats {
-       local_t         rx_dropped;
-       local_t         tx_dropped;
-       local_t         rx_nohandler;
-} __aligned(4 * sizeof(local_t));
+       unsigned long   rx_dropped;
+       unsigned long   tx_dropped;
+       unsigned long   rx_nohandler;
+} __aligned(4 * sizeof(unsigned long));
 
 #include <linux/cache.h>
 #include <linux/skbuff.h>
@@ -3843,15 +3843,15 @@ static __always_inline bool __is_skb_forwardable(const struct net_device *dev,
        return false;
 }
 
-struct net_device_core_stats *netdev_core_stats_alloc(struct net_device *dev);
+struct net_device_core_stats __percpu *netdev_core_stats_alloc(struct net_device *dev);
 
-static inline struct net_device_core_stats *dev_core_stats(struct net_device *dev)
+static inline struct net_device_core_stats __percpu *dev_core_stats(struct net_device *dev)
 {
        /* This READ_ONCE() pairs with the write in netdev_core_stats_alloc() */
        struct net_device_core_stats __percpu *p = READ_ONCE(dev->core_stats);
 
        if (likely(p))
-               return this_cpu_ptr(p);
+               return p;
 
        return netdev_core_stats_alloc(dev);
 }
@@ -3859,14 +3859,11 @@ static inline struct net_device_core_stats *dev_core_stats(struct net_device *de
 #define DEV_CORE_STATS_INC(FIELD)                                              \
 static inline void dev_core_stats_##FIELD##_inc(struct net_device *dev)                \
 {                                                                              \
-       struct net_device_core_stats *p;                                        \
+       struct net_device_core_stats __percpu *p;                               \
                                                                                \
-       preempt_disable();                                                      \
        p = dev_core_stats(dev);                                                \
-                                                                               \
        if (p)                                                                  \
-               local_inc(&p->FIELD);                                           \
-       preempt_enable();                                                       \
+               this_cpu_inc(p->FIELD);                                         \
 }
 DEV_CORE_STATS_INC(rx_dropped)
 DEV_CORE_STATS_INC(tx_dropped)

diff --git a/net/core/dev.c b/net/core/dev.c
index 8c6c08446556a23baae724d263bb9bd63eb39c7e..1461c2d9dec8099a9a2d43a704b4c6cb0375f480 100644 (file)
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -10304,7 +10304,7 @@ void netdev_stats_to_stats64(struct rtnl_link_stats64 *stats64,
 }
 EXPORT_SYMBOL(netdev_stats_to_stats64);
 
-struct net_device_core_stats *netdev_core_stats_alloc(struct net_device *dev)
+struct net_device_core_stats __percpu *netdev_core_stats_alloc(struct net_device *dev)
 {
        struct net_device_core_stats __percpu *p;
 
@@ -10315,11 +10315,7 @@ struct net_device_core_stats *netdev_core_stats_alloc(struct net_device *dev)
                free_percpu(p);
 
        /* This READ_ONCE() pairs with the cmpxchg() above */
-       p = READ_ONCE(dev->core_stats);
-       if (!p)
-               return NULL;
-
-       return this_cpu_ptr(p);
+       return READ_ONCE(dev->core_stats);
 }
 EXPORT_SYMBOL(netdev_core_stats_alloc);
 
@@ -10356,9 +10352,9 @@ struct rtnl_link_stats64 *dev_get_stats(struct net_device *dev,
 
                for_each_possible_cpu(i) {
                        core_stats = per_cpu_ptr(p, i);
-                       storage->rx_dropped += local_read(&core_stats->rx_dropped);
-                       storage->tx_dropped += local_read(&core_stats->tx_dropped);
-                       storage->rx_nohandler += local_read(&core_stats->rx_nohandler);
+                       storage->rx_dropped += READ_ONCE(core_stats->rx_dropped);
+                       storage->tx_dropped += READ_ONCE(core_stats->tx_dropped);
+                       storage->rx_nohandler += READ_ONCE(core_stats->rx_nohandler);
                }
        }
        return storage;