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

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Date: Mon, 9 Oct 2023 16:36:42 +0800
From: Yajun Deng <yajun.deng@...ux.dev>
To: Eric Dumazet <edumazet@...gle.com>
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 2023/10/9 16:20, Eric Dumazet wrote:
> 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.


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))++;