Messages in this thread | | | Subject | Re: [RFC, v2] powerpc/powernv: Introduce kernel param to control fastsleep workaround behavior | From | Benjamin Herrenschmidt <> | Date | Tue, 17 Mar 2015 20:39:58 +1100 |
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On Tue, 2015-03-17 at 19:57 +1100, Michael Ellerman wrote: > > So my first preference is that you just bite the bullet and decide to either > always apply the workaround, or just stick with the current behaviour. That's a > trade-off between (I think) better idle latency but a risk of checkstops, vs. > slower idle latency but less (how much less?) risk of checkstops. > > I think the reason you're proposing a kernel parameter is because we aren't > willing to make that decision, ie. we're saying that users should decide. Is > that right?
Correct. More specifically, a fairly high profile user that I will not name here has expressed interest in such a feature...
> I'm not a big fan of kernel parameters. They are a pain to use, and are often > just pushing a decision down one layer for no reason. What I mean is that > individual users are probably just going to accept whatever the default value > is from their distro.
Right. This is quite an obscure tunable.
> But anyway, that's a bit of a rant. > > As far as this patch is concerned, I don't think it actually needs to be a > kernel parameter. > > >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.
> 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.
I wouldn't bother with NOP'ing in that case, a runtime test will probably be noise in the measurement.
Cheers, Ben.
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