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

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Date:   Mon, 26 Mar 2018 12:57:02 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     rostedt <rostedt@...dmis.org>, Alexei Starovoitov <ast@...com>
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>,
        "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 12:35 PM, rostedt rostedt@...dmis.org wrote:

> On Mon, 26 Mar 2018 09:25:07 -0700
> Alexei Starovoitov <ast@...com> wrote:
> 
>> commit log of patch 6 states:
>> 
>> "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."
>> 
>> and in patch 7:
>> 
>> +static void *__find_tp(struct tracepoint *tp, void *priv)
>> +{
>> +       char *name = priv;
>> +
>> +       if (!strcmp(tp->name, name))
>> +               return tp;
>> +       return NULL;
>> +}
>> ...
>> +       struct tracepoint *tp;
>> ...
>> +       tp = for_each_kernel_tracepoint(__find_tp, tp_name);
>> +       if (!tp)
>> +               return -ENOENT;
>> 
>> still not obvious?
> 
> Please just create a new function called tracepoint_find_by_name(), and
> use that. I don't see any benefit in using a for_each* function for
> such a simple routine. Not to mention, you then don't need to know the
> internals of a tracepoint in kernel/bpf/syscall.c.

Steven's approach is fine by me, considering there should never be duplicated
tracepoint definitions (it emits a __tracepoint_##name symbol which would cause
multiple symbols definition errors at link time if there are more than
a single definition per tracepoint name in the core kernel). The exported
API should probably be named "kernel_tracepoint_find_by_name()" or something
similar, thus indicating that it only lookup tracepoints in the core kernel.

Which brings the next question: what are Alexei's plan to handle tracepoints
in modules, considering module load/unload scenarios ? The tracepoint API
has module notifiers for this, but it does not appear to be used in this
patch series.

Thanks,

Mathieu

> 
> -- Steve

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