Messages in this thread | | | Date | Tue, 3 Nov 2009 11:53:14 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [RELEASE] Userspace RCU 0.3.0 |
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* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > On Tue, Nov 03, 2009 at 10:02:34AM -0500, Mathieu Desnoyers wrote: > > Hi everyone, > > > > I released userspace RCU 0.3.0, which includes a small API change for > > the "deferred work" interface. After discussion with Paul, I decided to > > drop the support for call_rcu() and only provide defer_rcu(), to make > > sure I don't provide an API with the same name as the kernel RCU but > > with different arguments and semantic. It will generate the following > > linker error if used: > > > > file.c:240: undefined reference to > > `__error_call_rcu_not_implemented_please_use_defer_rcu' > > > > Note that defer_rcu() should *not* be used in RCU read-side C.S., > > because it calls synchronize_rcu() if the queue is full. This is a major > > distinction from call_rcu(). (note to self: eventually we should add > > some self-check code to detect defer_rcu() nested within RCU read-side > > C.S.). > > > > I plan to eventually implement a proper call_rcu() within the userspace > > RCU library. It's not, however, a short-term need for me at the moment. > > I can tell that we need to get you going on some real-time work. ;-) >
:-)
> (Sorry, but I really couldn't resist!) >
It's true that it becomes important when real-time behavior is required at the call_rcu() execution site. However, even typical use of call_rcu() has some limitations in this area: in a scenario where the struct rcu_head passed to call_rcu() is allocated dynamically, kmalloc and friends do not offer any kind of wait-free/lock-free guarantees. So the way call_rcu() works is to push the burden of RT impact on the original struct rcu_head allocation. But I agree that it makes out-of-memory/queue full error handling much easier, because all the allocation is done at the same site.
The main disadvantage of the call_rcu() approach though is that I cannot see any clean way to perform call_rcu() rate-limitation on a per-cpu basis. This would basically imply that we have to stop providing RT call_rcu() at some point to ensure we do not go over a certain threshold.
A possible solution would be to make call_rcu() return an error when it goes over some threshold. The caller would have to deal with the error, possibly by rejecting the whole operation (so maybe another CPU/cloud node could take over the work). This seems cleaner than delaying execution of the call_rcu() site. The caller could actually decide to either reject the whole operation or to delay its execution.
Mathieu
> Thanx, Paul
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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