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
