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Message-ID: <c43a3dde-fa4d-4a87-6f96-397813db5bd6@linux.dev>
Date:   Thu, 28 Sep 2023 23:40:19 +0800
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Subject: Re: [PATCH v6] net/core: Introduce netdev_core_stats_inc()
Content-Language: en-US
To:     Eric Dumazet <edumazet@google.com>
Cc:     davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        Alexander Lobakin <aleksander.lobakin@intel.com>
References: <20230928100418.521594-1-yajun.deng@linux.dev>
 <CANn89iL9uy58ZrZRPEtrvQ7ckv5hVTq8shx3OesQA6SWoUOP=g@mail.gmail.com>
From:   Yajun Deng <yajun.deng@linux.dev>
In-Reply-To: <CANn89iL9uy58ZrZRPEtrvQ7ckv5hVTq8shx3OesQA6SWoUOP=g@mail.gmail.com>
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On 2023/9/28 22:18, Eric Dumazet wrote:
> On Thu, Sep 28, 2023 at 12:04 PM Yajun Deng <yajun.deng@linux.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@linux.dev>
>> Suggested-by: Alexander Lobakin <aleksander.lobakin@intel.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);