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

[Eric Dumazet]

On Mon, Oct 9, 2023 at 10:36 AM Yajun Deng <yajun.deng@xxxxxxxxx> wrote:
>
>
> On 2023/10/9 16:20, Eric Dumazet wrote:
> > On Mon, Oct 9, 2023 at 10:14 AM Yajun Deng <yajun.deng@xxxxxxxxx> wrote:
> >>
> >> On 2023/10/9 15:53, Eric Dumazet wrote:
> >>> On Mon, Oct 9, 2023 at 5:07 AM Yajun Deng <yajun.deng@xxxxxxxxx> wrote:
> >>>
> >>>> 'this_cpu_read + this_cpu_write' and 'pr_info + this_cpu_inc' will make
> >>>> the trace work well.
> >>>>
> >>>> They all have 'pop' instructions in them. This may be the key to making
> >>>> the trace work well.
> >>>>
> >>>> Hi all,
> >>>>
> >>>> I need your help on percpu and ftrace.
> >>>>
> >>> I do not think you made sure netdev_core_stats_inc() was never inlined.
> >>>
> >>> Adding more code in it is simply changing how the compiler decides to
> >>> inline or not.
> >>
> >> Yes, you are right. It needs to add the 'noinline' prefix. The
> >> disassembly code will have 'pop'
> >>
> >> instruction.
> >>
> > The function was fine, you do not need anything like push or pop.
> >
> > The only needed stuff was the call __fentry__.
> >
> > The fact that the function was inlined for some invocations was the
> > issue, because the trace point
> > is only planted in the out of line function.
>
>
> But somehow the following code isn't inline? They didn't need to add the
> 'noinline' prefix.
>
> +               field = (unsigned long *)((void *)this_cpu_ptr(p) + offset);
> +               WRITE_ONCE(*field, READ_ONCE(*field) + 1);
>
> Or
> +               (*(unsigned long *)((void *)this_cpu_ptr(p) + offset))++;
>

I think you are very confused.

You only want to trace netdev_core_stats_inc() entry point, not
arbitrary pieces of it.