linux-kernel - Re: [PATCH v4 16/27] tracing: Remove regular RCU context for

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Message-ID: <65796626.20397.1583528124078.JavaMail.zimbra@efficios.com>
Date:   Fri, 6 Mar 2020 15:55:24 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     rostedt <rostedt@...dmis.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        paulmck <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        dan carpenter <dan.carpenter@...cle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH v4 16/27] tracing: Remove regular RCU context for
 _rcuidle tracepoints (again)

----- On Mar 6, 2020, at 3:45 PM, rostedt rostedt@...dmis.org wrote:

> On Fri, 6 Mar 2020 15:22:46 -0500 (EST)
> Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> 
>> 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.
> 
> I thought about this approach, but thought it was a bit overkill. As the
> kernel doesn't have an internal API, I figured we can switch this over to
> flags when we get another flag to add. Unless you can think of one in the
> near future.

The additional feature I have in mind for near future would be to register
a probe which can take a page fault to a "sleepable" tracepoint. This would
require preemption to be enabled and use of SRCU.

We can always change things when we get there.

Thanks,

Mathieu

> 
>> 
>> 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)
> 
> No, as soon as there is another boolean to add, the rcu version would be
> switched to flags. I even thought about making the rcu and prio into one,
> and change prio to be a SHRT_MAX max, and have the other 16 bits be for
> flags.
> 
> -- Steve
> 
> 
>> 
> > which seems rather error-prone and less readable than a set of flags.

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