linux-kernel - Re: [PATCH net-next v7] net/core: Introduce netdev_core_stats

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Date:   Mon, 9 Oct 2023 10:20:25 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Yajun Deng <yajun.deng@...ux.dev>
Cc:     rostedt@...dmis.org, mhiramat@...nel.org, dennis@...nel.org,
        tj@...nel.org, cl@...ux.com, mark.rutland@....com,
        davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alexander Lobakin <aleksander.lobakin@...el.com>,
        linux-trace-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH net-next v7] net/core: Introduce netdev_core_stats_inc()

On Mon, Oct 9, 2023 at 10:14 AM Yajun Deng <yajun.deng@...ux.dev> wrote:
>
>
> On 2023/10/9 15:53, Eric Dumazet wrote:
> > On Mon, Oct 9, 2023 at 5:07 AM Yajun Deng <yajun.deng@...ux.dev> 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.