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

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Date:   Mon, 20 Nov 2023 14:18:29 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        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>, 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 10:47:42PM +0100, Peter Zijlstra wrote:
> 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 ?

Tasks Trace RCU avoids SRCU's full barriers and the array accesses in the
read-side primitives.  This can be important when tracing low-overhead
components of fast paths.

							Thanx, Paul