Re: 2.6.16-rt10 crash on ppc

[Nick Piggin]

Ingo Molnar wrote:

> * Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
>
> > I'm not very familiar with the -rt tree, but possibly what should be 
> > happening, if interrupts are executed in process context and allowed 
> > to schedule, is that their memory allocations should also be allowed 
> > to reclaim memory.
>
> indeed - very good point. Emin, could you try the patch below [which 
> isnt a full solution but should be a good first approximation], does it 
> make any difference?
>
>
> > OTOH, I guess for a deterministic realtime system, you need to 
> > allocate fixed minimum amounts of memory for each element of the 
> > system so you never run out like this.
>
> yeah, preallocation is pretty much the only way to go for deterministic 
> workloads. Still, networking (and other complex subsystems) can still be 
> used in parallel to deterministic tasks - and those should not be 
> starved easier when PREEMPT_RT is enabled. In fact, with the patch below 
> it could become much more robust - we could in fact achieve to never 
> fail an allocation due to being in an atomic context.

Yes, that patch is basically what I had in mind.

Is -rt ever allocating memory from really-hard-don't-preempt-me context?
I guess not, unless the zone->lock is one of those locks too, right?

Should you add a

 #else
    BUG_ON(_really_dont_preempt_me());
 #endif

just for safety, or will such misusage get caught elsewhere (eg. when
attempting to take zone->lock).

Thanks,
Nick

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