linux-kernel - Re: [PATCH v4 1/5] tracing: Introduce faultable tracepoints

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Date:   Mon, 20 Nov 2023 22:47:42 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        linux-kernel@...r.kernel.org,
        Michael Jeanson <mjeanson@...icios.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Yonghong Song <yhs@...com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <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@...r.kernel.org,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH v4 1/5] tracing: Introduce faultable tracepoints

On Mon, Nov 20, 2023 at 03:54:14PM -0500, Mathieu Desnoyers wrote:
> When invoked from system call enter/exit instrumentation, accessing
> user-space data is a common use-case for tracers. However, tracepoints
> currently disable preemption around iteration on the registered
> tracepoint probes and invocation of the probe callbacks, which prevents
> tracers from handling page faults.
> 
> Extend the tracepoint and trace event APIs to allow defining a faultable
> tracepoint which invokes its callback with preemption enabled.
> 
> Also extend the tracepoint API to allow tracers to request specific
> probes to be connected to those faultable tracepoints. When the
> TRACEPOINT_MAY_FAULT flag is provided on registration, the probe
> callback will be called with preemption enabled, and is allowed to take
> page faults. Faultable probes can only be registered on faultable
> tracepoints and non-faultable probes on non-faultable tracepoints.
> 
> The tasks trace rcu mechanism is used to synchronize read-side
> marshalling of the registered probes with respect to faultable probes
> unregistration and teardown.

What is trace-trace rcu and why is it needed here? What's wrong with
SRCU ?