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

[Yajun Deng]

On 2023/10/9 18:16, Eric Dumazet wrote:
> On Mon, Oct 9, 2023 at 11:43 AM Yajun Deng <yajun.deng@xxxxxxxxx> wrote:
> > On 2023/10/9 17:30, Eric Dumazet wrote:
> > > 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.
> >
> > Yes, I will trace netdev_core_stats_inc() entry point. I mean to replace
> >
> > +                                       field = (__force unsigned long
> > __percpu *)((__force void *)p + offset);
> > +                                       this_cpu_inc(*field);
> >
> > with
> >
> > +               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))++;
> >
> > The netdev_core_stats_inc() entry point will work fine even if it doesn't
> > have 'noinline' prefix.
> >
> > I don't know why this code needs to add 'noinline' prefix.
> > +               field = (__force unsigned long __percpu *)((__force void *)p + offset);
> > +               this_cpu_inc(*field);
> C compiler decides to inline or not, depending on various factors.
>
> The most efficient (and small) code is generated by this_cpu_inc()
> version, allowing the compiler to inline it.
>
> If you copy/paste this_cpu_inc()  twenty times, then the compiler
> would  not inline the function anymore.


Got it. Thank you.