Re: [this_cpu_xx V5 19/19] SLUB: Experimental new fastpath w/ointerrupt disable

[Mathieu Desnoyers]

* Christoph Lameter (cl@xxxxxxxxxxxxxxxxxxxx) wrote:
> On Mon, 12 Oct 2009, Mathieu Desnoyers wrote:
> 
> > > If I just enable interrupts there then the preempt check will not be
> > > done and we may miss a scheduling point.
> > >
> >
> > That's why you should do:
> >
> > local_irq_save()
> > preempt_enable_no_resched()
> > ...
> > local_irq_restore()
> > preempt_check_resched()
> 
> What is the difference then to
> 
> local_irq_save()
> 
> ...
> 
> local_irq_enable();
> preempt_enable();
> 
> ?

local_irq_save()
preempt_enable_no_resched()  <- barrier()
...
local_irq_enable()
preempt_check_resched()

vs

local_irq_save()
...
local_irq_enable();
preempt_enable(); <- barrier()

In the first scenario, the compiler barrier is at the beginning of the
slow path function, which should impose less restrictions on the compiler
optimizations.

Mathieu


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Mathieu Desnoyers
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