Re: Preempt? (was Re: Cannot enable DMA on SATA drive (SCSI-libsata,VIA SATA))

[Nick Piggin]

Jeff Garzik wrote:
> Roland Dreier wrote:
>
> >     Jeff> I strongly recommend disabling kernel preemption.  It is a
> >     Jeff> hack that hides bugs.
> >
> > Why do you say that?  Preempt seems to be the cleanest way to low
> > latency, and if anything it exposes locking bugs and races rather than
> > hiding anything.
>
>
> Clean?  Hardly.  It breaks up code paths that were never written to be 
> broken up.

But lots of things change. Unserialising the kernel broke code paths.

>  The proper fix is to locate and fix high latency code paths. 
>  Preempt does nothing but hide those high latency code paths, and 
> discourage people from seeking a better solution.
>
> Fix the drivers, rather than bandaid over them with preempt.
>
> If all code paths in the kernel were low latency, then you would not 
> need preempt at all.

When you say low latency, you mean small lock hold times, *and*
cond_rescheds placed everywhere - it is this second requirement
that isn't the cleanest way of doign things.

With preempt, sure you still need small lock hold times. No big
deal.
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