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?
