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

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Date:   Wed, 28 Mar 2018 15:22:24 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     rostedt <rostedt@...dmis.org>
Cc:     Alexei Starovoitov <ast@...com>,
        "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 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.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com