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Date:   Tue, 21 Nov 2023 09:40:24 -0500
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Subject: Re: [PATCH v4 1/5] tracing: Introduce faultable tracepoints
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To:     Peter Zijlstra <peterz@infradead.org>
Cc:     "Paul E. McKenney" <paulmck@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
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        Michael Jeanson <mjeanson@efficios.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Yonghong Song <yhs@fb.com>, Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>, bpf@vger.kernel.org,
        Joel Fernandes <joel@joelfernandes.org>
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From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
In-Reply-To: <20231121143647.GI8262@noisy.programming.kicks-ass.net>
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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?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com