Messages in this thread | | | Date | Mon, 22 Nov 2004 13:25:54 -0800 | Subject | Re: Priority Inheritance Test (Real-Time Preemption) | From | Bill Huey (hui) <> |
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On Mon, Nov 22, 2004 at 01:37:41PM +0100, Ingo Molnar wrote: > in the -RT patchset one of the reasons why i've gone for the completely > preemptible variant is to trigger all priority inversion problems > outright. In the first variant they didnt really trigger - but they were > present. Once the locks were almost all preemptible, PI problems > surfaced in a big way - causing people to report them and forcing me to > fix them :-) > > There are lots of critical sections in Linux and we cannot design around > them - so if the goal is hard-RT properties and latencies then priority > inversion is a problem that has to be solved. Later on we could easily > revert some of the hw-related spinlocks to raw spinlocks, and/or the > known-O(1) critical sections as well.
Good. Yeah, the only piont I was making is not to cover up contention problems, which are kernel performance problems with PI. That's all.
> the paper cited is not very persuasive to me though. It lists problems > of an incomplete/incorrect PI implementation, and comes to the IMO false > (and unrelated) conclusion that somehow PI-handling is not desired.
Yeah, I agree. PI has it's place, but the main point of the paper is that getting a really good PI protocol is a very difficult thing and might not be worth it if you can use other techniques. What you do with that assertion is situational.
A number of those complaints, as I see it, don't apply to Linux because of how it's avoided things like deadlocking, fine grainedness, etc... are done. But he does, IMO, outline the difficulty of getting a decent PI implementation.
> Obviously PI makes only sense if it's implemented correctly. I think i > managed to fix the problems Esben's testsuite uncovered, in the current > -RT patch. Anyway, this implementation is also special in that it relies > on correct SMP locking of Linux:
I'll check it out.
> i dont have any intentions to turn Linux into a 'priority inheritance > world'. PI handling is only a property of the PREEMPT_RT feature > intended for the most latency-sensitive applications - the main and > primary critical-section model of Linux is and should still be a healthy > mix of spinlocks and mutexes. Having only mutexes (or only spinlocks) is > an extreme that _does_ hurt the common case. PREEMPT_RT 'only' lives on > the back of SMP-Linux.
Yeah, that's my point. The reason why/if/when Linux will be strong at RT is because of the SMP work.
bill
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