Re: [RFC, v2] powerpc/powernv: Introduce kernel param to control fastsleep workaround behavior

[Michael Ellerman <mpe@ellerman.id.au>]

On Tue, 2015-03-17 at 21:19 +0530, Shreyas B Prabhu wrote:
> 
> On Tuesday 17 March 2015 03:09 PM, Benjamin Herrenschmidt wrote:
> > On Tue, 2015-03-17 at 19:57 +1100, Michael Ellerman wrote:
> >>
> >> >From what I can see below, the decision as to whether you apply the workaround
> >> or not doesn't affect the list of idle states. So this could just as well be a
> >> runtime parameter, ie. a sysfs file, which can then be set by the user whenever
> >> they like? They might do it in a boot script, but that's up to them.
> > 
> > Right, that would work too.
> 
> Okay. I'll send a patch with this design.

Thanks.

> >> For simplicity I think it would also be fine to make it a write-once parameter,
> >> ie. you don't need to handle undoing it.
> > 
> > It would be easy enough to make it rw using stop machine I think... 
> > 
> >> I think the only complication that would add is that you'd need to be a little
> >> careful about the order in which you nop out the calls vs applying the
> >> workaround, in case some threads are idle when you're called.
> 
> Right, we should be safe with this sequence-
> - NOP call to undo workaround
> - Apply workaround on all cores.
> - NOP call to apply workaround
 
Yeah that sounds right.

> > I wouldn't bother with NOP'ing in that case, a runtime test will probably be noise
> > in the measurement.
> 
> Didn't get your point here. Do you mean, ignore the request if some
> cores are in sleep or deeper state?

I *think* what he means is we probably don't actually need to patch a nop
in/out. Instead we could just test a flag, because the cost of testing a flag
is miniscule compared to the rest of the logic.

cheers