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

Yajun Deng
2023-10-08T09:13:23Z

On 2023/10/8 16:53, Eric Dumazet wrote:
> On Sun, Oct 8, 2023 at 10:44 AM Yajun Deng <yajun.deng@linux.dev> wrote:
>>
>> On 2023/10/8 15:18, Eric Dumazet wrote:
>>> On Sun, Oct 8, 2023 at 9:00 AM Yajun Deng <yajun.deng@linux.dev> wrote:
>>>> On 2023/10/8 14:45, Eric Dumazet wrote:
>>>>> On Sat, Oct 7, 2023 at 8:34 AM Yajun Deng <yajun.deng@linux.dev> wrote:
>>>>>> On 2023/10/7 13:29, Eric Dumazet wrote:
>>>>>>> On Sat, Oct 7, 2023 at 7:06 AM 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.
>>>>>>>>
>>>>>>> ...
>>>>>>>
>>>>>>>> +
>>>>>>>> +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/e25b5f3c-bd97-56f0-de86-b93a3172870d@linux.dev/
>>>>>>
>>>>>>> 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:
>>>>> I honestly do not know what you are talking about.
>>>>>
>>>>> Have you even tried to change your patch to use
>>>>>
>>>>> field = (__force unsigned long __percpu *)((__force void *)p + offset);
>>>>> this_cpu_inc(*field);
>>>> Yes, I tested this code. But the following couldn't show anything even
>>>> if the rx_dropped is increasing.
>>>>
>>>> 'sudo python3 /usr/share/bcc/tools/trace netdev_core_stats_inc'
>>> Well, I am not sure about this, "bpftrace" worked for me.
>>>
>>> Make sure your toolchain generates something that looks like what I got:
>>>
>>> 000000000000ef20 <netdev_core_stats_inc>:
>>> ef20: f3 0f 1e fa endbr64
>>> ef24: e8 00 00 00 00 call ef29 <netdev_core_stats_inc+0x9>
>>> ef25: R_X86_64_PLT32 __fentry__-0x4
>>> ef29: 55 push %rbp
>>> ef2a: 48 89 e5 mov %rsp,%rbp
>>> ef2d: 53 push %rbx
>>> ef2e: 89 f3 mov %esi,%ebx
>>> ef30: 48 8b 87 f0 01 00 00 mov 0x1f0(%rdi),%rax
>>> ef37: 48 85 c0 test %rax,%rax
>>> ef3a: 74 0b je ef47 <netdev_core_stats_inc+0x27>
>>> ef3c: 89 d9 mov %ebx,%ecx
>>> ef3e: 65 48 ff 04 08 incq %gs:(%rax,%rcx,1)
>>> ef43: 5b pop %rbx
>>> ef44: 5d pop %rbp
>>> ef45: c3 ret
>>> ef46: cc int3
>>> ef47: e8 00 00 00 00 call ef4c <netdev_core_stats_inc+0x2c>
>>> ef48: R_X86_64_PLT32 .text.unlikely.+0x13c
>>> ef4c: 48 85 c0 test %rax,%rax
>>> ef4f: 75 eb jne ef3c <netdev_core_stats_inc+0x1c>
>>> ef51: eb f0 jmp ef43 <netdev_core_stats_inc+0x23>
>>> ef53: 66 66 66 66 2e 0f 1f data16 data16 data16 cs nopw 0x0(%rax,%rax,1)
>>> ef5a: 84 00 00 00 00 00
>>
>> I'll share some I can see it.
>>
>> 1.
>>
>> objdump -D vmlinux
>>
>> ffffffff81b2f170 <netdev_core_stats_inc>:
>> ffffffff81b2f170: e8 8b ea 55 ff callq ffffffff8108dc00
>> <__fentry__>
>> ffffffff81b2f175: 55 push %rbp
>> ffffffff81b2f176: 48 89 e5 mov %rsp,%rbp
>> ffffffff81b2f179: 48 83 ec 08 sub $0x8,%rsp
>> ffffffff81b2f17d: 48 8b 87 e8 01 00 00 mov 0x1e8(%rdi),%rax
>> ffffffff81b2f184: 48 85 c0 test %rax,%rax
>> ffffffff81b2f187: 74 0d je ffffffff81b2f196
>> <netdev_core_stats_inc+0x26>
>> ffffffff81b2f189: 89 f6 mov %esi,%esi
>> ffffffff81b2f18b: 65 48 ff 04 30 incq %gs:(%rax,%rsi,1)
>> ffffffff81b2f190: c9 leaveq
>> ffffffff81b2f191: e9 aa 31 6d 00 jmpq ffffffff82202340
>> <__x86_return_thunk>
>> ffffffff81b2f196: 89 75 fc mov %esi,-0x4(%rbp)
>> ffffffff81b2f199: e8 82 ff ff ff callq ffffffff81b2f120
>> <netdev_core_stats_alloc>
>> ffffffff81b2f19e: 8b 75 fc mov -0x4(%rbp),%esi
>> ffffffff81b2f1a1: 48 85 c0 test %rax,%rax
>> ffffffff81b2f1a4: 75 e3 jne ffffffff81b2f189
>> <netdev_core_stats_inc+0x19>
>> ffffffff81b2f1a6: c9 leaveq
>> ffffffff81b2f1a7: e9 94 31 6d 00 jmpq ffffffff82202340
>> <__x86_return_thunk>
>> ffffffff81b2f1ac: 0f 1f 40 00 nopl 0x0(%rax)
>>
>>
>> 2.
>>
>> sudo cat /proc/kallsyms | grep netdev_core_stats_inc
>>
>> ffffffff9c72f120 T netdev_core_stats_inc
>> ffffffff9ca2676c t netdev_core_stats_inc.cold
>> ffffffff9d5235e0 r __ksymtab_netdev_core_stats_inc
>>
>>
>> 3.
>>
>> ➜ ~ ifconfig enp34s0f0
>> enp34s0f0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
>> inet 10.10.30.88 netmask 255.255.255.0 broadcast 10.10.30.255
>> inet6 fe80::6037:806c:14b6:f1ca prefixlen 64 scopeid 0x20
>> ether 04:d4:c4:5c:81:42 txqueuelen 1000 (Ethernet)
>> RX packets 29024 bytes 3118278 (3.1 MB)
>> RX errors 0 dropped 794 overruns 0 frame 0
>> TX packets 16961 bytes 2662290 (2.6 MB)
>> TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
>> device interrupt 29 memory 0x39fff4000000-39fff47fffff
>>
>> ➜ ~ ifconfig enp34s0f0
>> enp34s0f0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
>> inet 10.10.30.88 netmask 255.255.255.0 broadcast 10.10.30.255
>> inet6 fe80::6037:806c:14b6:f1ca prefixlen 64 scopeid 0x20
>> ether 04:d4:c4:5c:81:42 txqueuelen 1000 (Ethernet)
>> RX packets 29272 bytes 3148997 (3.1 MB)
>> RX errors 0 dropped 798 overruns 0 frame 0
>> TX packets 17098 bytes 2683547 (2.6 MB)
>> TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
>> device interrupt 29 memory 0x39fff4000000-39fff47fffff
>>
>>
>> The rx_dropped is increasing.
>>
>>
>> 4.
>>
>> sudo python3 /usr/share/bcc/tools/trace netdev_core_stats_inc
>>
>> TIME PID TID COMM FUNC
>>
>> (Empty, I didn't see anything.)
>>
>>
>> 5.
>>
>> sudo trace-cmd record -p function -l netdev_core_stats_inc
>>
>> sudo trace-cmd report
>>
>> (Empty, I didn't see anything.)
>>
>>
>> If I add a 'pr_info("\n");' like:
>>
>> + pr_info("\n");
>> field = (__force unsigned long __percpu *)((__force void *)p +
>> offset);
>> this_cpu_inc(*field);
>>
>>
>> Everything is OK. The 'pr_info("\n");' can be changed to anything else,
>> but not
>>
>> without it.
> This seems to be a bug that has nothing to do with the patch.
>
> Try getting help from Steven maybe.

Hi Steven,

Need your help.

1. The following code wouldn't trace anything by the command 'sudo
python3 /usr/share/bcc/tools/trace netdev_core_stats_inc'

+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);
+}
+EXPORT_SYMBOL_GPL(netdev_core_stats_inc);

2. If I add a 'pr_info("\n");', it would be fine. The 'pr_info("\n");' can be changed to others.

+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;
+ }
+ pr_info("\n");
+ field = (__force unsigned long __percpu *)((__force void *)p + offset);
+ this_cpu_inc(*field);
+}
+EXPORT_SYMBOL_GPL(netdev_core_stats_inc);

I don't know why we need to add something in netdev_core_stats_inc, the trace will be fine.