Re: [PATCH v6] net/core: Introduce netdev_core_stats_inc()

[Yajun Deng <yajun.deng@linux.dev>]

On 2023/9/28 23:44, Eric Dumazet wrote:
> On Thu, Sep 28, 2023 at 5:40 PM Yajun Deng <yajun.deng@linux.dev> wrote:
>>
>> 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.
> 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);