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

Eric Dumazet
2023-10-08T08:54:01Z

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.