Re: [PATCH v4 16/27] tracing: Remove regular RCU context for _rcuidle tracepoints (again)

Mathieu Desnoyers
2020-03-06T20:22:48Z

----- On Mar 6, 2020, at 12:55 PM, rostedt rostedt@goodmis.org wrote:

> On Fri, 6 Mar 2020 11:04:28 -0500 (EST)
> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
>
>> If we care about not adding those extra branches on the fast-path, there is
>> an alternative way to do things: BPF could provide two distinct probe callbacks,
>> one meant for rcuidle tracepoints (which would have the trace_rcu_enter/exit),
>> and
>> the other for the for 99% of the other callsites which have RCU watching.
>>
>> I would recommend performing benchmarks justifying the choice of one approach
>> over
>> the other though.
>
> I just whipped this up (haven't even tried to compile it), but this should
> satisfy everyone. Those that register a callback that needs RCU protection
> simply registers with one of the _rcu versions, and all will be done. And
> since DO_TRACE is a macro, and rcuidle is a constant, the rcu protection
> code will be compiled out for locations that it is not needed.
>
> With this, perf doesn't even need to do anything extra but register with
> the "_rcu" version.
>
> -- Steve
>

[...]

> diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
> index 73956eaff8a9..1797e20fd471 100644
> --- a/kernel/tracepoint.c
> +++ b/kernel/tracepoint.c
> @@ -295,6 +295,7 @@ static int tracepoint_remove_func(struct tracepoint *tp,
> * @probe: probe handler
> * @data: tracepoint data
> * @prio: priority of this function over other registered functions
> + * @rcu: set to non zero if the callback requires RCU protection
> *
> * Returns 0 if ok, error value on error.
> * Note: if @tp is within a module, the caller is responsible for
> @@ -302,8 +303,8 @@ static int tracepoint_remove_func(struct tracepoint *tp,
> * performed either with a tracepoint module going notifier, or from
> * within module exit functions.
> */
> -int tracepoint_probe_register_prio(struct tracepoint *tp, void *probe,
> - void *data, int prio)
> +int tracepoint_probe_register_prio_rcu(struct tracepoint *tp, void *probe,
> + void *data, int prio, int rcu)

I agree with the overall approach. Just a bit of nitpicking on the API:

I understand that the "prio" argument is a separate argument because it can take
many values. However, "rcu" is just a boolean, so I wonder if we should not rather
introduce a "int flags" with a bitmask enum, e.g.

int tracepoint_probe_register_prio_flags(struct tracepoint *tp, void *probe,
void *data, int prio, int flags)

where flags would be populated through OR between labels of this enum:

enum tracepoint_flags {
TRACEPOINT_FLAG_RCU = (1U << 0),
};

We can then be future-proof for additional flags without ending up calling e.g.

tracepoint_probe_register_featurea_featureb_featurec(tp, probe, data, 0, 1, 0, 1)

which seems rather error-prone and less readable than a set of flags.

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.