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Date: Thu, 28 Sep 2023 23:40:19 +0800
From: Yajun Deng <yajun.deng@...ux.dev>
To: Eric Dumazet <edumazet@...gle.com>
Cc: davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
Alexander Lobakin <aleksander.lobakin@...el.com>
Subject: Re: [PATCH v6] net/core: Introduce netdev_core_stats_inc()
On 2023/9/28 22:18, Eric Dumazet wrote:
> On Thu, Sep 28, 2023 at 12:04 PM Yajun Deng <yajun.deng@...ux.dev> 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@...ux.dev>
>> Suggested-by: Alexander Lobakin <aleksander.lobakin@...el.com>
>> ---
>> 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@linux.dev/
>> ---
>> 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.
>
> You might be able tweak this to
>
> unsigned long __percpu *field = (unsigned long __percpu) ((u8 *)p + offset);
> this_cpu_inc(field);
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