Messages in this thread | | | Date | Tue, 21 Nov 2023 09:56:55 -0500 | Subject | Re: [PATCH v4 1/5] tracing: Introduce faultable tracepoints | From | Mathieu Desnoyers <> |
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On 2023-11-21 09:46, Peter Zijlstra wrote: > 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?
Personally what I am looking for is a clear documentation of preemptible rcu with respect to whether it is possible to block on I/O (take a page fault, call schedule() explicitly) from within a preemptible rcu critical section. I guess this is a hard question because there is no clear statement to that effect in the kernel documentation.
If it is allowed (which I doubt), then I wonder about the effect of those long readers on grace period delays. Things like expedited grace periods may suffer.
Based on Documentation/RCU/rcu.rst:
Preemptible variants of RCU (CONFIG_PREEMPT_RCU) get the same effect, but require that the readers manipulate CPU-local counters. These counters allow limited types of blocking within RCU read-side critical sections. SRCU also uses CPU-local counters, and permits general blocking within RCU read-side critical sections. These variants of RCU detect grace periods by sampling these counters.
Then we just have to find a definition of "limited types of blocking" vs "general blocking".
Thanks,
Mathieu
-- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com
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