linux-kernel - Re: [RFC PATCH] tracepoint: Provide tracepoint_kernel_find_by

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Date:   Tue, 27 Mar 2018 08:28:53 -0700
From:   "Joel Fernandes (Google)" <joel.opensrc@...il.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     rostedt <rostedt@...dmis.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC PATCH] tracepoint: Provide tracepoint_kernel_find_by_name

On Tue, Mar 27, 2018 at 6:27 AM, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
<snip>
>>> +static void find_tp(struct tracepoint *tp, void *priv)
>>> +{
>>> +       struct tp_find_args *args = priv;
>>> +
>>> +       if (!strcmp(tp->name, args->name)) {
>>> +               WARN_ON_ONCE(args->tp);
>>> +               args->tp = tp;
>>
>> I think this runtime check is not needed if it really can't happen
>> (linker verifies that already as you mentioned) although I'm not
>> opposed if you still want to keep it, because there's no way to break
>> out of the outer loop anyway so I guess your call..
>
> We can change the outer loop and break from it if needed, that's not
> an issue.
>
>> I would just do:
>>
>> if (args->tp)
>>    return;
>>
>> if find_tp already found the tracepoint.
>>
>> Tried to also create a duplicate tracepoint and I get:
>>  CC      init/version.o
>>  AR      init/built-in.o
>>  AR      built-in.o
>>  LD      vmlinux.o
>> block/blk-core.o:(__tracepoints+0x440): multiple definition of
>> `__tracepoint_sched_switch'
>> kernel/sched/core.o:(__tracepoints+0x440): first defined here
>> Makefile:1032: recipe for target 'vmlinux' failed
>> make: *** [vmlinux] Error 1
>
> Yeah, as I state in my changelog, I'm very well aware that the linker
> is able to catch those. This was the purpose of emitting a
> __tracepoint_##name symbol in the tracepoint definition macro. This
> WARN_ON_ONCE() is a redundant check for an invariant that we expect
> is provided by the linker. Given that it's not a fast path, I would
> prefer to keep this redundant check in place, given that an average
> factor 2 speedup on a slow path should really not be something we
> need to optimize for. IMHO, for slow paths, robustness is more important
> than speed (unless the slow path becomes so slow that it really affects
> the user).
>
> I envision that a way to break this invariant would be to compile a
> kernel object with gcc -fvisibility=hidden or such. I admit this is
> particular scenario is really far fetched, but it illustrates why I
> prefer to keep an extra safety net at runtime for linker-based
> validations when possible.
>
> If a faster tracepoint lookup function is needed, we should consider
> perfect hashing algorithms done post-build, but so far nobody has
> complained about speed of this lookup operation. Anyhow a factor 2 in
> the speed of this lookup should really not matter, right ?

Yes, I agree with all the great reasons. So lets do it your way then.

thanks,

- Joel