Subject: Re: [PATCH net-next v7] net/core: Introduce netdev_core_stats_inc()

From: Yajun Deng <[email-masked]>
Date: Sat, 7 Oct 2023 14:34:11 +0800

On 2023/10/7 13:29, Eric Dumazet wrote:
> On Sat, Oct 7, 2023 at 7:06 AM Yajun Deng <[email-masked]> 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.
>>
> ...
>
>> +
>> +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);
>> +       unsigned long *field;
>> +
>> +       if (unlikely(!p))
>> +               p = netdev_core_stats_alloc(dev);
>> +
>> +       if (p) {
>> +               field = (unsigned long *)((void *)this_cpu_ptr(p) + offset);
>> +               WRITE_ONCE(*field, READ_ONCE(*field) + 1);
> This is broken...
>
> As I explained earlier, dev_core_stats_xxxx(dev) can be called from
> many different contexts:
>
> 1) process contexts, where preemption and migration are allowed.
> 2) interrupt contexts.
>
> Adding WRITE_ONCE()/READ_ONCE() is not solving potential races.
>
> I _think_ I already gave you how to deal with this ?

Yes, I replied in v6.

https://lore.kernel.org/all/[email-masked]/

> Please try instead:
>
> +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);
> +       unsigned long __percpu *field;
> +
> +       if (unlikely(!p)) {
> +               p = netdev_core_stats_alloc(dev);
> +               if (!p)
> +                       return;
> +       }
> +       field = (__force unsigned long __percpu *)((__force void *)p + offset);
> +       this_cpu_inc(*field);
> +}

This wouldn't trace anything even the rx_dropped is in increasing. It 
needs to add an extra operation, such as:

pr_info, ++, trace_xxx... . I don't know what's going on.

If this is adopted, I need to send two patches, one is  introduce 
netdev_core_stats_inc, another is add an tracepoint , like:

+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);
+       unsigned long __percpu *field;
+
+       if (unlikely(!p)) {
+               p = netdev_core_stats_alloc(dev);
+               if (!p)
+                       return;
+       }
+	trace_netdev_core_stats_inc(dev, offset);
+       field = (__force unsigned long __percpu *)((__force void *)p + offset);
+       this_cpu_inc(*field);
+}

--- a/include/trace/events/net.h
+++ b/include/trace/events/net.h

+TRACE_EVENT(netdev_core_stats_inc,
+
+       TP_PROTO(struct net_device *dev,
+                u32 offset),
+
+       TP_ARGS(dev, offset),
+
+       TP_STRUCT__entry(
+               __string(       name,           dev->name )
+               __string(       driver, netdev_drivername(dev))
+               __field(        u32,            offset          )
+       ),
+
+       TP_fast_assign(
+               __assign_str(name, dev->name);
+               __assign_str(driver, netdev_drivername(dev));
+               __entry->offset = offset;
+       ),
+
+       TP_printk("dev=%s driver=%s offset=%u",
+               __get_str(name), __get_str(driver), __entry->offset)
+);

We can trace netdev_core_stats_inc by tracepoint or kprobe.