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

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Message-ID: <CAADnVQKot7kEYsEQrEszGeTuug4fpWGkc4GKA_yNeFi6OHe3uw@mail.gmail.com>
Date:   Fri, 6 Mar 2020 15:10:17 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     rostedt <rostedt@...dmis.org>,
        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 Fri, Mar 6, 2020 at 12:55 PM Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
> ----- 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.

I'm working on sleepable bpf as well and this extra flag for tracepoints
would come very handy, so I would go with flags approach right away.
We wouldn't need to touch the same protos multiple times,
less conflicts for us all, etc.