Re: [block IO crash] Re: 2.6.39-rc5-git2 boot crashs

[Ingo Molnar]

* Christoph Lameter <cl@xxxxxxxxx> wrote:

> On Thu, 5 May 2011, Tejun Heo wrote:
> 
> > Anyways, that's what I've been thinking.  I'll get to it in the next
> > devel cycle or the one after that.  What do you guys think about soft
> > irq masking idea?
> 
> Great idea. Would make the whole irq on/off business much cheaper.

The tradeoffs are *not at all* clear and the result (on x86) is not
'much cheaper', at all ...

In particular the irq-enable path gets complicated by the need to check the 
flag and call a hardirq handling function in that case - a far cry from the 
single-byte POPF instruction. It will be somewhat cheaper cycle-wise - but the 
code gets bloated, so the instruction cache impact has to be measured 
carefully. (See my other mail for details.)

There's also the fact that PUSHF+CLI+POPF sequence has been getting cheaper all 
the time with newer hardware generations. CLI+STI is even cheaper, 10 cycles 
both on Intel and AMD CPUs. So it's an optimization that might get narrower and 
narrower with every CPU generation.

Thanks,

	Ingo
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