netdev - Re: [PATCH v5 bpf-next 06/10] tracepoint: compute num

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Message-ID: <523311773.184.1522079745421.JavaMail.zimbra@efficios.com>
Date:   Mon, 26 Mar 2018 11:55:45 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Alexei Starovoitov <ast@...com>
Cc:     rostedt <rostedt@...dmis.org>,
        "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>,
        "Frank Ch. Eigler" <fche@...hat.com>
Subject: Re: [PATCH v5 bpf-next 06/10] tracepoint: compute num_args at build
 time

----- On Mar 26, 2018, at 11:42 AM, Alexei Starovoitov ast@...com wrote:

> On 3/26/18 8:14 AM, Mathieu Desnoyers wrote:
>> ----- On Mar 26, 2018, at 11:02 AM, rostedt rostedt@...dmis.org wrote:
>>
>>> On Fri, 23 Mar 2018 19:30:34 -0700
>>> Alexei Starovoitov <ast@...com> wrote:
>>>
>>>> From: Alexei Starovoitov <ast@...nel.org>
>>>>
>>>> add fancy macro to 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.
>>>>
>>>> for_each_tracepoint_range() api has no users inside the kernel.
>>>> Make it more useful with ability to stop for_each() loop depending
>>>> via callback return value.
>>>> In such form it's used in subsequent patch.
>>>
>>> I believe this is used by LTTng.
>>
>> Indeed, and by SystemTAP as well.
>>
>> What justifies the need to stop mid-iteration ? A less intrusive alternative
>> would be to use the "priv" data pointer to keep state telling further calls
>> to return immediately. Does performance of iteration over tracepoints really
>> matter here so much that stopping iteration immediately is worth it ?
> 
> I'm sure both you and Steven are not serious when you object
> to _in-tree_ change to for_each_kernel_tracepoint() that
> affects _out-of_tree_ modules?
> 
> Just change your module to 'return NULL' instead of plain 'return'.

I never said I objected to adapt the LTTng out of tree code. If there is a
solid reason for changing the kernel API, I will adapt my code to those
changes.

What I'm trying to understand here is whether there is solid ground for
the added complexity you are proposing. Is it a performance enhancement ?
If so, explanation of the use cases targeted, and numbers that measure
performance improvements are needed.

How is your patch making tracepoints "more useful" ?

Thanks,

Mathieu


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