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:
> 
> > > In the middle of the function we have determine that we have to go to the
> > > page allocator to get more memory. There is not much the compiler can do
> > > to speed that up.
> >
> > Indeed, the compiler cannot do much about it. However, the programer
> > (you) can move the preempt_enable_no_resched() part of the
> > preempt_enable() to the beginning of the function.
> 
> Ok but then we have the issue that the later irq enable in the
> slowpath will not check for preemption.
> 
> > > > I still recommend the preempt_enable_no_resched() at the beginning of
> > > > __slab_alloc(), and simply putting a check_resched() here (which saves
> > > > us the odd compiler barrier in the middle of function).
> > >
> > > Then preemption would be unnecessarily disabled for the page allocator
> > > call?
> >
> > No ?
> > preempt_enable_no_resched() enables preemption.
> 
> 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()

Mathieu



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Mathieu Desnoyers
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