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

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Date:   Tue, 21 Nov 2023 15:36:47 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        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 09:06:18AM -0500, Mathieu Desnoyers wrote:
> Task trace RCU fits a niche that has the following set of requirements/tradeoffs:
> 
> - Allow page faults within RCU read-side (like SRCU),
> - Has a low-overhead read lock-unlock (without the memory barrier overhead of SRCU),
> - The tradeoff: Has a rather slow synchronize_rcu(), but tracers should not care about
>   that. Hence, this is not meant to be a generic replacement for SRCU.
> 
> Based on my reading of https://lwn.net/Articles/253651/ , preemptible RCU is not a good
> fit for the following reasons:
> 
> - It disallows blocking within a RCU read-side on non-CONFIG_PREEMPT kernels,

Your counter points are confused, we simply don't build preemptible RCU
unless PREEMPT=y, but that could surely be fixed and exposed as a
separate flavour.

> - AFAIU the mmap_sem used within the page fault handler does not have priority inheritance.

What's that got to do with anything?

Still utterly confused about what task-tracing rcu is and how it is
different from preemptible rcu.