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SubjectRe: [this_cpu_xx V5 19/19] SLUB: Experimental new fastpath w/o interrupt disable
* Christoph Lameter (cl@linux-foundation.org) 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


--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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