netdev - Re: [PATCH v7 bpf-next 06/10] tracepoint: compute num

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Date:   Wed, 28 Mar 2018 12:25:44 -0700
From:   Alexei Starovoitov <ast@...com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rostedt <rostedt@...dmis.org>
CC:     "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        netdev <netdev@...r.kernel.org>,
        kernel-team <kernel-team@...com>,
        linux-api <linux-api@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v7 bpf-next 06/10] tracepoint: compute num_args at build
 time

On 3/28/18 12:22 PM, Mathieu Desnoyers wrote:
>
> ----- On Mar 28, 2018, at 2:54 PM, rostedt rostedt@...dmis.org wrote:
>
>> On Wed, 28 Mar 2018 11:19:34 -0700
>> Alexei Starovoitov <ast@...com> wrote:
>>
>>> On 3/28/18 11:10 AM, Steven Rostedt wrote:
>>>> On Wed, 28 Mar 2018 11:03:24 -0700
>>>> Alexei Starovoitov <ast@...com> wrote:
>>>>
>>>>> I can live with this overhead if Mathieu insists,
>>>>> but I prefer to keep it in 'struct tracepoint'.
>>>>>
>>>>> Thoughts?
>>>>
>>>> I'm fine with keeping it as is. We could probably use it for future
>>>> enhancements in perf and ftrace.
>>>>
>>>> Perhaps, we should just add a:
>>>>
>>>> #ifdef CONFIG_BPF_EVENTS
>>>>
>>>> Around the use cases of num_args.
>>>
>>> it sounds like a good idea, but implementation wise
>>> it will be ifdef CONFIG_BPF_EVENTS around u32 num_args;
>>> in struct tracepoint and similar double definition of
>>> DEFINE_TRACE_FN. One that uses num_args to init
>>> struct tracepoint and one that doesn't ?
>>> Feels like serious uglification of already macros heavy code.
>>> Also what it will address?
>>
>> 32bit bloat ;-)
>>
>> But I agree, it's not worth uglifying it.
>>
>> -- Steve
>>
>>> cache hot/cold argument clearly doesn't apply.
>
> In the current situation I'm fine with adding this extra field
> to struct tracepoint. However, we should keep in mind to move
> all non-required cache-cold fields to a separate section at
> some point. Clearly just this single field won't make a difference
> due to other fields and padding.

Submitted v8 where num_args is moved to bpf side.
Please ack it.