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

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Message-ID: <67c5f9b4-b24a-2806-e8d6-8b5241c66d6e@fb.com>
Date:   Wed, 28 Mar 2018 09:43:56 -0700
From:   Alexei Starovoitov <ast@...com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC:     "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        rostedt <rostedt@...dmis.org>, netdev <netdev@...r.kernel.org>,
        kernel-team <kernel-team@...com>,
        linux-api <linux-api@...r.kernel.org>
Subject: Re: [PATCH v7 bpf-next 06/10] tracepoint: compute num_args at build
 time

On 3/28/18 6:49 AM, Mathieu Desnoyers wrote:
> ----- On Mar 27, 2018, at 10:11 PM, Alexei Starovoitov ast@...com wrote:
>
>> From: Alexei Starovoitov <ast@...nel.org>
>>
>> compute number of arguments passed into tracepoint
>> at compile time and store it as part of 'struct tracepoint'.
>> The number is necessary to check safety of bpf program access that
>> is coming in subsequent patch.
>
>
> Hi Alexei,
>
> Given that only eBPF needs this parameter count, we can move
> it to the struct bpf_raw_event_map newly introduced by Steven,
> right ? This would reduce bloat of struct tracepoint. For instance,
> we don't need to keep this count around when eBPF is configured
> out.

Makes sense. That is indeed cleaner. Will respin.

What I don't like though is 'bloat' argument.
'u32 num_args' padded to 8-byte takes exactly the same amount
of space in 'struct tracepoint' and in 'struct bpf_raw_event_map'
The number of these structures is the same as well
and chances that tracepoints are on while bpf is off are slim.
More so few emails ago you said:
"I'm perfectly fine with adding the "num_args" stuff. I think it's
really useful. It's only the for_each_kernel_tracepoint change for
which I'm trying to understand the rationale."

I'm guessing now you found out that num_args is not useful to lttng
and it bloats its data structures?
It's ok to change an opinion and I'm completely fine using that as
a real reason.
Will repsin, as I said. No problem.