Re: Real-Time Preemption and RCU

[Bill Huey (hui) <bhuey@lnxw.com>]

On Fri, Mar 18, 2005 at 05:55:44PM +0100, Esben Nielsen wrote:
> On Fri, 18 Mar 2005, Ingo Molnar wrote:
> > i really have no intention to allow multiple readers for rt-mutexes. We
> > got away with that so far, and i'd like to keep it so. Imagine 100
> > threads all blocked in the same critical section (holding the read-lock)
> > when a highprio writer thread comes around: instant 100x latency to let
> > all of them roll forward. The only sane solution is to not allow
> > excessive concurrency. (That limits SMP scalability, but there's no
> > other choice i can see.)
> 
> Unless a design change is made: One could argue for a semantics where
> write-locking _isn't_ deterministic and thus do not have to boost all the

RCU isn't write deterministic like typical RT apps are we can... (below :-))

> readers. Readers boost the writers but not the other way around. Readers
> will be deterministic, but not writers.
> Such a semantics would probably work for a lot of RT applications
> happening not to take any write-locks - these will in fact perform better. 
> But it will give the rest a lot of problems.

Just came up with an idea after I thought about how much of a bitch it
would be to get a fast RCU multipule reader semantic (our current shared-
exclusive lock inserts owners into a sorted priority list per-thread which
makes it very expensive for a simple RCU case since they are typically very
small batches of items being altered). Basically the RCU algorithm has *no*
notion of writer priority and to propagate a PI operation down all reader
is meaningless, so why not revert back to the original rwlock-semaphore to
get the multipule reader semantics ?

A notion of priority across a quiescience operation is crazy anyways, so
it would be safe just to use to the old rwlock-semaphore "in place" without
any changes or priorty handling addtions. The RCU algorithm is only concerned
with what is basically a coarse data guard and it isn't time or priority
critical.

What do you folks think ? That would make Paul's stuff respect multipule
readers which reduces contention and gets around the problem of possibly
overloading the current rt lock implementation that we've been bitching
about. The current RCU development track seem wrong in the first place and
this seem like it could be a better and more complete solution to the problem.

If this works, well, you heard it here first. :)

bill