Re: [PATCH v6] net/core: Introduce netdev_core_stats_inc()
From: Yajun Deng
Date: Thu Sep 28 2023 - 12:17:01 EST
On 2023/9/28 23:44, Eric Dumazet wrote:
On Thu, Sep 28, 2023 at 5:40 PM Yajun Deng <yajun.deng@xxxxxxxxx> wrote:
On 2023/9/28 22:18, Eric Dumazet wrote:
On Thu, Sep 28, 2023 at 12:04 PM Yajun Deng <yajun.deng@xxxxxxxxx> wrote:
Although there is a kfree_skb_reason() helper function that can be used to
find the reason why this skb is dropped, but most callers didn't increase
one of rx_dropped, tx_dropped, rx_nohandler and rx_otherhost_dropped.
For the users, people are more concerned about why the dropped in ip
is increasing.
Introduce netdev_core_stats_inc() for trace the caller of the dropped
skb. Also, add __code to netdev_core_stats_alloc(), as it's called
unlinkly.
Signed-off-by: Yajun Deng <yajun.deng@xxxxxxxxx>
Suggested-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
---
v6: merge netdev_core_stats and netdev_core_stats_inc together
v5: Access the per cpu pointer before reach the relevant offset.
v4: Introduce netdev_core_stats_inc() instead of export dev_core_stats_*_inc()
v3: __cold should be added to the netdev_core_stats_alloc().
v2: use __cold instead of inline in dev_core_stats().
v1: https://lore.kernel.org/netdev/20230911082016.3694700-1-yajun.deng@xxxxxxxxx/
---
include/linux/netdevice.h | 21 ++++-----------------
net/core/dev.c | 17 +++++++++++++++--
2 files changed, 19 insertions(+), 19 deletions(-)
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 7e520c14eb8c..eb1fa04fbccc 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -4002,32 +4002,19 @@ static __always_inline bool __is_skb_forwardable(const struct net_device *dev,
return false;
}
-struct net_device_core_stats __percpu *netdev_core_stats_alloc(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 p;
-
- return netdev_core_stats_alloc(dev);
-}
+void netdev_core_stats_inc(struct net_device *dev, u32 offset);
#define DEV_CORE_STATS_INC(FIELD) \
static inline void dev_core_stats_##FIELD##_inc(struct net_device *dev) \
{ \
- struct net_device_core_stats __percpu *p; \
- \
- p = dev_core_stats(dev); \
- if (p) \
- this_cpu_inc(p->FIELD); \
Note that we were using this_cpu_inc() which implied :
- IRQ safety, and
- a barrier paired with :
net/core/dev.c:10548: storage->rx_dropped +=
READ_ONCE(core_stats->rx_dropped);
net/core/dev.c:10549: storage->tx_dropped +=
READ_ONCE(core_stats->tx_dropped);
net/core/dev.c:10550: storage->rx_nohandler +=
READ_ONCE(core_stats->rx_nohandler);
net/core/dev.c:10551: storage->rx_otherhost_dropped
+= READ_ONCE(core_stats->rx_otherhost_dropped);
+ netdev_core_stats_inc(dev, \
+ offsetof(struct net_device_core_stats, FIELD)); \
}
DEV_CORE_STATS_INC(rx_dropped)
DEV_CORE_STATS_INC(tx_dropped)
DEV_CORE_STATS_INC(rx_nohandler)
DEV_CORE_STATS_INC(rx_otherhost_dropped)
+#undef DEV_CORE_STATS_INC
static __always_inline int ____dev_forward_skb(struct net_device *dev,
struct sk_buff *skb,
diff --git a/net/core/dev.c b/net/core/dev.c
index 606a366cc209..88a32c392c1d 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -10497,7 +10497,8 @@ void netdev_stats_to_stats64(struct rtnl_link_stats64 *stats64,
}
EXPORT_SYMBOL(netdev_stats_to_stats64);
-struct net_device_core_stats __percpu *netdev_core_stats_alloc(struct net_device *dev)
+static __cold struct net_device_core_stats __percpu *netdev_core_stats_alloc(
+ struct net_device *dev)
{
struct net_device_core_stats __percpu *p;
@@ -10510,7 +10511,19 @@ struct net_device_core_stats __percpu *netdev_core_stats_alloc(struct net_device
/* This READ_ONCE() pairs with the cmpxchg() above */
return READ_ONCE(dev->core_stats);
}
-EXPORT_SYMBOL(netdev_core_stats_alloc);
+
+void netdev_core_stats_inc(struct net_device *dev, u32 offset)
+{
+ /* 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 (unlikely(!p))
+ p = netdev_core_stats_alloc(dev);
+
+ if (p)
+ (*(unsigned long *)((void *)this_cpu_ptr(p) + offset))++;
While here you are using a ++ operation that :
- is not irq safe
- might cause store-tearing.
I would suggest a preliminary patch converting the "unsigned long" fields in
struct net_device_core_stats to local_t
Do you mean it needs to revert the commit 6510ea973d8d ("net: Use
this_cpu_inc() to increment
net->core_stats") first? But it would allocate memory which breaks on
PREEMPT_RT.
I think I provided an (untested) alternative.
unsigned long __percpu *field = (__force unsigned long __percpu *)
((__force u8 *)p + offset);
this_cpu_inc(field);
unsigned long __percpu *field = (__force unsigned long __percpu *)
((__force u8 *)p + offset);
this_cpu_inc(*(int *)field);
This would compiler success. But I didn't test it.
This cold look complex.
Shoud I base v3? Export dev_core_stats_*_inc() intead of introduce netdev_core_stats_inc().
That would be easy.
You might be able tweak this to
unsigned long __percpu *field = (unsigned long __percpu) ((u8 *)p + offset);
this_cpu_inc(field);