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

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Date:   Tue, 21 Nov 2023 15:46:43 +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 09:40:24AM -0500, Mathieu Desnoyers wrote:
> On 2023-11-21 09:36, Peter Zijlstra wrote:
> > 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.
> 
> In addition to taking the mmap_sem, the page fault handler need to block
> until its requested pages are faulted in, which may depend on disk I/O.
> Is it acceptable to wait for I/O while holding preemptible RCU read-side?

I don't know, preemptible rcu already needs to track task state anyway,
it needs to ensure all tasks have passed through a safe spot etc.. vs regular
RCU which only needs to ensure all CPUs have passed through start.

Why is this such a hard question?