Re: [RFC] maximum latency tracking infrastructure

[Arjan van de Ven]

Nick Piggin wrote:
> Arjan van de Ven wrote:
> > Jesse Barnes wrote:
> >
> > > On Thursday, August 24, 2006 10:41 am, Arjan van de Ven wrote:
> > >
> > > > The reason for adding this infrastructure is that power management in
> > > > the idle loop needs to make a tradeoff between latency and power
> > > > savings (deeper power save modes have a longer latency to running code
> > > > again).
> > >
> > >
> > > What if a processor was already in a sleep state when a call to 
> > > set_acceptable_latency() latency occurs? 
> >
> >
> > there's nothing sane that can be done in that case; any wake up 
> > already will cause the unwanted latency!
> > A premature wakeup is only making it happen *now*, but now is as 
> > inconvenient a time as any...
> > (in fact it may be a worst case time scenario, say, an audio 
> > interrupt...)
>
> Surely you would call set_acceptable_latency() *before* running such
> operation that requires the given latency? And that set_acceptable_latency
> would block the caller until all CPUs are set to wake within this latency.
>
> That would be the API semantics I would expect, anyway.

but that means it blocks, and thus can't be used in irq context

(the usage model I imagine happens most is a set_acceptable_latency() which can block during device init,
with either no or a very course limit, and a modify_acceptable_latency(), which cannot block, from irq context or
device open)
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