Re: [patch] Voluntary Kernel Preemption, 2.6.12-rc4-mm2

[Ingo Molnar <mingo@elte.hu>]

* Nick Piggin <nickpiggin@yahoo.com.au> wrote:

> >remove it if it's not being used, but right now the only choice that 
> >distributions have is no preemption or full-blown CONFIG_PREEMPT. Ask 
> >the kernel maintainers at SuSE why they havent enabled CONFIG_PREEMPT in 
> >their kernels.
> >
> 
> I guess it is a number of reasons. Probably the main one had 
> traditionally been the chance of bugs. I guess the next big one is 
> return on overhead (ie. the scheduling latency soon runs into the 
> problem of long critical sections), although thanks to you and others, 
> I understand that is becoming less and less of an issue over time too.
> 
> If a new SUSE kernel branch was started from 2.6.12 with VP turned on 
> rather than PREEMPT then I would probably argue against it a little 
> bit ;)

dont think of scheduling latencies as a binary thing a'ka "do we have 
good preemption latencies". It's a continuum, with almost a continuum 
number of techniques. One thing is sure: close to one end of the 
spectrum we have PREEMPT_NONE, and pretty close to the other end of the 
spectrum we have PREEMPT_RT.

both PREEMPT_VOLUNTARY and CONFIG_PREEMPT are at arbitrary points within 
that continuum, with different cost/benefit tradeoffs. Neither is 
perfect, and both are 'ugly' in the theoretical sense.

now, i dont intend to populate our .config with a continuum number of 
preemption models ;) But clearly the past 4 years have shown that no 
major distro was brave enough to go CONFIG_PREEMPT, so a solution 
inbetween is needed. -VP is precisely such a (very low-impact) solution.  
It has a ridiculously low impact:

 include/linux/kernel.h | 18 +++++++++++----

we already talked an order of magnitude more about this feature than its 
size is (with help text included :). Lets go with it and let people know 
that the water is fine. If it's unused it can be zapped easily.

	Ingo
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