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

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Message-ID: <20231121160300.GK8262@noisy.programming.kicks-ass.net>
Date:   Tue, 21 Nov 2023 17:03:00 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.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 Tue, Nov 21, 2023 at 07:58:40AM -0800, Paul E. McKenney wrote:

> Tasks Trace RCU allows general blocking in its readers, not just the
> subject-to-priority-boosting blocking permitted within preemptible RCU
> readers.  Restrictions on the use of Tasks Trace RCU are in place to allow
> getting away with this general blocking.  Even systems generously endowed
> with memory are not going to do well when the RCU grace period is blocked
> on I/O, especially if that I/O is across a network to a slow file server.
> 
> Which means a separate RCU instance is needed.  Which is Tasks Trace RCU.

Separate instance not a problem, nor really the question.

What is the basic mechanism of task-tracing? Is it really the existing
tasks-rcu extended with read-side critical sections and call_rcu ?

If so, then why not have it be tasks-rcu?

Or is it a variant of the preemptible/SRCU class of RCUs that are
counter-array based? I suspect not.

So once again, what exactly is tasks-tracing ?