linux-kernel - Re: [RFC PATCH 6/6] tracing: use sched-RCU instead of SRCU for rcuidle tracepoints

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Date:   Mon, 26 Oct 2020 16:44:59 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Michael Jeanson <mjeanson@...icios.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Yonghong Song <yhs@...com>, paulmck <paulmck@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, acme <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [RFC PATCH 6/6] tracing: use sched-RCU instead of SRCU for
 rcuidle tracepoints

On Mon, 26 Oct 2020 10:28:07 -0400 (EDT)
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> I agree with Peter. Removing the trace_.*_rcuidle weirdness from the tracepoint
> API and fixing all callers to ensure they trace from a context where RCU is
> watching would simplify instrumentation of the Linux kernel, thus making it harder
> for subtle bugs to hide and be unearthed only when tracing is enabled. This is

Note, the lockdep RCU checking of a tracepoint is outside of it being
enabled or disable. So if a non rcuidle() tracepoint is in a location that
RCU is not watching, it will complain loudly, even if you don't enable that
tracepoint.

> AFAIU the general approach Thomas Gleixner has been aiming for recently, and I
> think it is a good thing.
> 
> So if we consider this our target, and that the current state of things is that
> we need to have RCU watching around callback invocation, then removing the
> dependency on SRCU seems like an overall simplification which does not regress
> feature-wise nor speed-wise compared with what we have upstream today. The next
> steps would then be to audit all rcuidle tracepoints and make sure the context
> where they are placed has RCU watching already, so we can remove the tracepoint

Just remove the _rcuidle() from them, and lockdep will complain if they are
being called without RCU watching.

-- Steve


> rcuidle API. That would effectively remove the calls to rcu_irq_{enter,exit}_irqson
> from the tracepoint code.
> 
> This is however beyond the scope of the proposed patch set.
> 
> Thanks,
> 
> Mathieu
>