Re: Real-Time Preemption and RCU

[Paul E. McKenney]

On Fri, Mar 18, 2005 at 08:49:03AM +0100, Ingo Molnar wrote:
> 
> * Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> 
> > Seems to me that it would be good to have an RCU implementation that
> > meet the requirements of the Real-Time Preemption patch, but that is
> > 100% compatible with the "classic RCU" API.  Such an implementation
> > must meet a number of requirements, which are listed at the end of
> > this message (search for "REQUIREMENTS").
> 
> [ Wow. you must be a secret telepatic mind-reader - yesterday i was
>   thinking about mailing you, because my approach to RCU preemptability
>   (the API variants) clearly sucked and caused problems all around, both
>   in terms of maintainability and in terms of stability and
>   scalability. ]

Wish I could claim that, but must credit dumb luck.  ;-)

> > 5.	The final version, which both scales and meets realtime
> > 	requirements, as well as exactly matching the "classic RCU"
> > 	API.
> > 
> > I have tested this approach, but in user-level scaffolding.  All of
> > these implementations should therefore be regarded with great
> > suspicion: untested, probably don't even compile.  Besides which, I
> > certainly can't claim to fully understand the real-time preempt patch,
> > so I am bound to have gotten something wrong somewhere.  In any case,
> > none of these implementations are a suitable replacement for "classic
> > RCU" on large servers, since they acquire locks in the RCU read-side
> > critical sections. However, they should scale enough to support small
> > SMP systems, inflicting only a modest performance penalty.
> 
> basically for PREEMPT_RT the only constraint is that RCU sections should
> be preemptable. Whatever the performance cost. If PREEMPT_RT is merged
> into the upstream kernel then it will (at least initially) be at a
> status similar to NOMMU: it will be tolerated as long as it causes no
> 'drag' on the main code. The RCU API variants i introduced clearly
> violated this requirement, and were my #1 worry wrt. upstream
> mergability.

Fair enough!

> > I believe that implementation #5 is most appropriate for real-time
> > preempt kernels. [...]
> 
> yeah, agreed - it looks perfect - both the read and write side is
> preemptable. Can i just plug the code you sent into rcupudate.c and
> expect it to work, or would you like to send a patch? If you prefer you
> can make it an unconditional patch against an upstream kernel to keep
> things simple for you - i'll then massage it to be properly PREEMPT_RT
> dependent.

I will give it a shot, but feel free to grab anything at any point,
starting either from the patch I will send in a bit or from my earlier
email.  Do whatever works best for you.

For the patch, here are my questions:

o	What is the best way to select between classic RCU and this
	scheme?

	1.	Massive #ifdef across rcupdate.c

	2.	Create an rcupdate_rt.c and browbeat the build system
		into picking one or the other (no clue if this is
		possible...)

	3.	Create an rcupdate_rt.c and rely on the linker to pick
		one or the other, with rcupdate.h generating different
		external symbol names to make the choice.

	Left to myself, I would grit my teeth and do #1, since I
	understand how to do it.  Flames welcome, as long as accompanied
	by good advice!

o	How best to interface to OOM?  Left to myself, I leave this
	for later.  ;-)

I will take the cowardly approach of patching against the upstream kernel.

> > [...] In theory, #3 might be appropriate, but if I understand the
> > real-time preempt implementation of reader-writer lock, it will not
> > perform well if there are long RCU read-side critical sections, even
> > in UP kernels.
> 
> all RCU-locked sections must be preemptable in -RT.  Basically RCU is a
> mainstream API that is used by lots of code and will be introduced in
> many other areas as well. From the -RT kernel's POV sees this as an
> 'uncontrollable latency source', which keeps introducing critical
> sections. One major goal of PREEMPT_RT is to convert all popular
> critical section APIs into preemptible sections, so that the amount of
> code that is non-preemptable is drastically reduced and can be managed
> (and thus can be trusted). This goal has a higher priority than any
> performance consideration, because it doesnt matter what performance you
> have, if you cannot trust the kernel to be deterministic.

Makes sense to me!

						Thanx, Paul
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