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

[Nick Piggin <nickpiggin@yahoo.com.au>]

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.