Re: [PATCH net-next v4] net/core: Introduce netdev_core_stats_inc() for trace

[Paolo Abeni]

On Mon, 2023-09-18 at 10:40 +0800, Yajun Deng 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. Also, move dev_core_stats()
> and netdev_core_stats_alloc() to dev.c, as they are not called externally.
> 
> Signed-off-by: Yajun Deng <yajun.deng@xxxxxxxxx>
> Suggested-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
> ---
> 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            | 26 ++++++++++++++++++++++++--
>  2 files changed, 28 insertions(+), 19 deletions(-)
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 0896aaa91dd7..cddd4873b5b0 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -3980,32 +3980,19 @@ 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);						\
> +	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 ccff2b6ef958..f4cccdf05450 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -10475,7 +10475,8 @@ 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;
>  
> @@ -10488,7 +10489,28 @@ 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);
> +
> +static inline struct net_device_core_stats __percpu *netdev_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)
> +{
> +	struct net_device_core_stats __percpu *p;
> +
> +	p = netdev_core_stats(dev);
> +	if (p)
> +		this_cpu_inc(*(unsigned long *)((void *)p + offset));

The above is causing a lot of compile warning, as it's discarding the
(required) __percpu annotation.

You need to first access the per cpu pointer and then reach for the
relevant offset.

Cheers,

Paolo