linux-kernel - Re: [RFC] convert ftrace syscall tracer to TRACE

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Date:	Sat, 9 May 2009 16:02:45 +0200
From:	Frédéric Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Jason Baron <jbaron@...hat.com>, Tom Zanussi <tzanussi@...il.com>,
	linux-kernel@...r.kernel.org, laijs@...fujitsu.com,
	rostedt@...dmis.org, peterz@...radead.org,
	mathieu.desnoyers@...ymtl.ca, jiayingz@...gle.com,
	mbligh@...gle.com, roland@...hat.com, fche@...hat.com
Subject: Re: [RFC] convert ftrace syscall tracer to TRACE_EVENT()

Le 9 mai 2009 15:33, Ingo Molnar <mingo@...e.hu> a écrit :
>
> * Frédéric Weisbecker <fweisbec@...il.com> wrote:
>
>> > Secondly, we should reuse the information we get in
>> > SYSCALL_DEFINE, to construct the TRACE_EVENT tracepoints
>> > directly - without having to list all syscalls again in a
>> > separate file.
>>
>> Indeed, that's not trivial though, but feasible. I'm not sure we
>> can reuse the TRACE_EVENT macro directly inside SYSCALL_DEFINE.
>> The resulting macro tempest effect that would occur confuses me
>> and I have troubles to imagine the result.
>
> Lets take an example. This syscall:
>
> SYSCALL_DEFINE3(sched_setscheduler, pid_t, pid, int, policy,
>                struct sched_param __user *, param)
>
> Is equivalent to:
>
> SYSCALL_DEFINE3(name, t1, v1, t2, v2, t3, v3)
>
> ('t' for type, 'v' for variable/value).
>
> This would transform into the following TRACE_EVENT() construct:
>
> TRACE_EVENT_SYSCALL2():
>
> TRACE_EVENT(sys_##name,
>        TP_PROTO(t1 v1, t2 v2),
>        TP_ARGS(v1, v2),
>        TP_STRUCT__entry(
>                __field(t1, v1)
>                __field(t2, v2)
>        ),
>        TP_fast_assign(
>                __entry->v1 = v1;
>                __entry->v2 = v2;
>        ),
>        TP_printk("%016Lx %016Lx", (u64)__entry->v1, (u64)__entry->v2)
> );
>
> We need TRACE_EVENT_SYSCALL[123456] definitions, and that's it.



Yeah, no problem with that.
This is more about the headers dependency layer that I'm worrying.
I guess we should just try and see what happens :)


> The only place where we lose type information is the printk format -
> but that's not a big issue, as i'd expect the event record to be the
> main user of this.


Indeed, that's not a big issue.
We can also define the custom printer callback we are using in the syscall
tracer, and assign it without using tp_printk.


> [ In addition to this, we could extend DEFINE_SYSCALL[1..6] with a
>  (optional) format string definition field, and fill that in for
>  anything that matters. ]


Yeah,


> Note, this assumes that all syscall types can be described via
> __field() - i think that's correct. (we dont want to deref strings
> as they are untrusted, and there are no arrays in syscall
> parameters)


Yeah, but we can also define a __string_from_user(), should be trivial.


> Can you see any complication?


Just about the order of headers to include and headers dependencies....


>        Ingo
>
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