Messages in this thread | | | Date | Tue, 21 Nov 2023 09:40:24 -0500 | Subject | Re: [PATCH v4 1/5] tracing: Introduce faultable tracepoints | From | Mathieu Desnoyers <> |
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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
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